diff --git a/sherpa-onnx/csrc/CMakeLists.txt b/sherpa-onnx/csrc/CMakeLists.txt
index 3216af65..bef262f9 100644
--- a/sherpa-onnx/csrc/CMakeLists.txt
+++ b/sherpa-onnx/csrc/CMakeLists.txt
@@ -34,6 +34,7 @@ set(sources
   offline-transducer-model-config.cc
   offline-transducer-model.cc
   offline-transducer-modified-beam-search-decoder.cc
+  online-conformer-transducer-model.cc
   online-lm.cc
   online-lm-config.cc
   online-lstm-transducer-model.cc
@@ -52,6 +53,7 @@ set(sources
   parse-options.cc
   resample.cc
   slice.cc
+  stack.cc
   symbol-table.cc
   text-utils.cc
   transpose.cc
@@ -241,6 +243,7 @@ if(SHERPA_ONNX_ENABLE_TESTS)
     packed-sequence-test.cc
     pad-sequence-test.cc
     slice-test.cc
+    stack-test.cc
     transpose-test.cc
     unbind-test.cc
   )
diff --git a/sherpa-onnx/csrc/online-conformer-transducer-model.cc b/sherpa-onnx/csrc/online-conformer-transducer-model.cc
new file mode 100644
index 00000000..8584f0ec
--- /dev/null
+++ b/sherpa-onnx/csrc/online-conformer-transducer-model.cc
@@ -0,0 +1,279 @@
+// sherpa-onnx/csrc/online-conformer-transducer-model.cc
+//
+// Copyright (c)  2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#include "sherpa-onnx/csrc/online-conformer-transducer-model.h"
+
+#include <assert.h>
+
+#include <algorithm>
+#include <memory>
+#include <sstream>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <vector>
+
+#if __ANDROID_API__ >= 9
+#include "android/asset_manager.h"
+#include "android/asset_manager_jni.h"
+#endif
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+#include "sherpa-onnx/csrc/cat.h"
+#include "sherpa-onnx/csrc/macros.h"
+#include "sherpa-onnx/csrc/online-transducer-decoder.h"
+#include "sherpa-onnx/csrc/onnx-utils.h"
+#include "sherpa-onnx/csrc/text-utils.h"
+#include "sherpa-onnx/csrc/unbind.h"
+
+namespace sherpa_onnx {
+
+OnlineConformerTransducerModel::OnlineConformerTransducerModel(
+    const OnlineTransducerModelConfig &config)
+    : env_(ORT_LOGGING_LEVEL_WARNING),
+      config_(config),
+      sess_opts_{},
+      allocator_{} {
+  sess_opts_.SetIntraOpNumThreads(config.num_threads);
+  sess_opts_.SetInterOpNumThreads(config.num_threads);
+
+  {
+    auto buf = ReadFile(config.encoder_filename);
+    InitEncoder(buf.data(), buf.size());
+  }
+
+  {
+    auto buf = ReadFile(config.decoder_filename);
+    InitDecoder(buf.data(), buf.size());
+  }
+
+  {
+    auto buf = ReadFile(config.joiner_filename);
+    InitJoiner(buf.data(), buf.size());
+  }
+}
+
+#if __ANDROID_API__ >= 9
+OnlineConformerTransducerModel::OnlineConformerTransducerModel(
+    AAssetManager *mgr, const OnlineTransducerModelConfig &config)
+    : env_(ORT_LOGGING_LEVEL_WARNING),
+      config_(config),
+      sess_opts_{},
+      allocator_{} {
+  sess_opts_.SetIntraOpNumThreads(config.num_threads);
+  sess_opts_.SetInterOpNumThreads(config.num_threads);
+
+  {
+    auto buf = ReadFile(mgr, config.encoder_filename);
+    InitEncoder(buf.data(), buf.size());
+  }
+
+  {
+    auto buf = ReadFile(mgr, config.decoder_filename);
+    InitDecoder(buf.data(), buf.size());
+  }
+
+  {
+    auto buf = ReadFile(mgr, config.joiner_filename);
+    InitJoiner(buf.data(), buf.size());
+  }
+}
+#endif
+
+void OnlineConformerTransducerModel::InitEncoder(void *model_data,
+                                                 size_t model_data_length) {
+  encoder_sess_ = std::make_unique<Ort::Session>(env_, model_data,
+                                                 model_data_length, sess_opts_);
+
+  GetInputNames(encoder_sess_.get(), &encoder_input_names_,
+                &encoder_input_names_ptr_);
+
+  GetOutputNames(encoder_sess_.get(), &encoder_output_names_,
+                 &encoder_output_names_ptr_);
+
+  // get meta data
+  Ort::ModelMetadata meta_data = encoder_sess_->GetModelMetadata();
+  if (config_.debug) {
+    std::ostringstream os;
+    os << "---encoder---\n";
+    PrintModelMetadata(os, meta_data);
+    SHERPA_ONNX_LOGE("%s", os.str().c_str());
+  }
+
+  Ort::AllocatorWithDefaultOptions allocator;  // used in the macro below
+  SHERPA_ONNX_READ_META_DATA(num_encoder_layers_, "num_encoder_layers");
+  SHERPA_ONNX_READ_META_DATA(T_, "T");
+  SHERPA_ONNX_READ_META_DATA(decode_chunk_len_, "decode_chunk_len");
+  SHERPA_ONNX_READ_META_DATA(left_context_, "left_context");
+  SHERPA_ONNX_READ_META_DATA(encoder_dim_, "encoder_dim");
+  SHERPA_ONNX_READ_META_DATA(pad_length_, "pad_length");
+  SHERPA_ONNX_READ_META_DATA(cnn_module_kernel_, "cnn_module_kernel");
+}
+
+void OnlineConformerTransducerModel::InitDecoder(void *model_data,
+                                                 size_t model_data_length) {
+  decoder_sess_ = std::make_unique<Ort::Session>(env_, model_data,
+                                                 model_data_length, sess_opts_);
+
+  GetInputNames(decoder_sess_.get(), &decoder_input_names_,
+                &decoder_input_names_ptr_);
+
+  GetOutputNames(decoder_sess_.get(), &decoder_output_names_,
+                 &decoder_output_names_ptr_);
+
+  // get meta data
+  Ort::ModelMetadata meta_data = decoder_sess_->GetModelMetadata();
+  if (config_.debug) {
+    std::ostringstream os;
+    os << "---decoder---\n";
+    PrintModelMetadata(os, meta_data);
+    SHERPA_ONNX_LOGE("%s", os.str().c_str());
+  }
+
+  Ort::AllocatorWithDefaultOptions allocator;  // used in the macro below
+  SHERPA_ONNX_READ_META_DATA(vocab_size_, "vocab_size");
+  SHERPA_ONNX_READ_META_DATA(context_size_, "context_size");
+}
+
+void OnlineConformerTransducerModel::InitJoiner(void *model_data,
+                                                size_t model_data_length) {
+  joiner_sess_ = std::make_unique<Ort::Session>(env_, model_data,
+                                                model_data_length, sess_opts_);
+
+  GetInputNames(joiner_sess_.get(), &joiner_input_names_,
+                &joiner_input_names_ptr_);
+
+  GetOutputNames(joiner_sess_.get(), &joiner_output_names_,
+                 &joiner_output_names_ptr_);
+
+  // get meta data
+  Ort::ModelMetadata meta_data = joiner_sess_->GetModelMetadata();
+  if (config_.debug) {
+    std::ostringstream os;
+    os << "---joiner---\n";
+    PrintModelMetadata(os, meta_data);
+    SHERPA_ONNX_LOGE("%s", os.str().c_str());
+  }
+}
+
+std::vector<Ort::Value> OnlineConformerTransducerModel::StackStates(
+    const std::vector<std::vector<Ort::Value>> &states) const {
+  int32_t batch_size = static_cast<int32_t>(states.size());
+
+  std::vector<const Ort::Value *> attn_vec(batch_size);
+  std::vector<const Ort::Value *> conv_vec(batch_size);
+
+  for (int32_t i = 0; i != batch_size; ++i) {
+    assert(states[i].size() == 2);
+    attn_vec[i] = &states[i][0];
+    conv_vec[i] = &states[i][1];
+  }
+
+  Ort::Value attn = Cat(allocator_, attn_vec, 2);
+  Ort::Value conv = Cat(allocator_, conv_vec, 2);
+
+  std::vector<Ort::Value> ans;
+  ans.reserve(2);
+  ans.push_back(std::move(attn));
+  ans.push_back(std::move(conv));
+
+  return ans;
+}
+
+std::vector<std::vector<Ort::Value>>
+OnlineConformerTransducerModel::UnStackStates(
+    const std::vector<Ort::Value> &states) const {
+  const int32_t batch_size =
+    states[0].GetTensorTypeAndShapeInfo().GetShape()[2];
+  assert(states.size() == 2);
+
+  std::vector<std::vector<Ort::Value>> ans(batch_size);
+
+  std::vector<Ort::Value> attn_vec = Unbind(allocator_, &states[0], 2);
+  std::vector<Ort::Value> conv_vec = Unbind(allocator_, &states[1], 2);
+
+  assert(attn_vec.size() == batch_size);
+  assert(conv_vec.size() == batch_size);
+
+  for (int32_t i = 0; i != batch_size; ++i) {
+    ans[i].push_back(std::move(attn_vec[i]));
+    ans[i].push_back(std::move(conv_vec[i]));
+  }
+
+  return ans;
+}
+
+std::vector<Ort::Value> OnlineConformerTransducerModel::GetEncoderInitStates() {
+  // Please see
+  // https://github.com/k2-fsa/icefall/blob/86b0db6eb9c84d9bc90a71d92774fe2a7f73e6ab/egs/librispeech/ASR/pruned_transducer_stateless5/conformer.py#L203
+  // for details
+  constexpr int32_t kBatchSize = 1;
+  std::array<int64_t, 4> h_shape{
+    num_encoder_layers_, left_context_, kBatchSize, encoder_dim_};
+  Ort::Value h = Ort::Value::CreateTensor<float>(allocator_, h_shape.data(),
+                                                 h_shape.size());
+
+  Fill<float>(&h, 0);
+
+  std::array<int64_t, 4> c_shape{num_encoder_layers_, cnn_module_kernel_ - 1,
+                                 kBatchSize, encoder_dim_};
+
+  Ort::Value c = Ort::Value::CreateTensor<float>(allocator_, c_shape.data(),
+                                                 c_shape.size());
+
+  Fill<float>(&c, 0);
+
+  std::vector<Ort::Value> states;
+
+  states.reserve(2);
+  states.push_back(std::move(h));
+  states.push_back(std::move(c));
+
+  return states;
+}
+
+std::pair<Ort::Value, std::vector<Ort::Value>>
+OnlineConformerTransducerModel::RunEncoder(Ort::Value features,
+                                           std::vector<Ort::Value> states,
+                                           Ort::Value processed_frames) {
+  std::array<Ort::Value, 4> encoder_inputs = {
+      std::move(features),
+      std::move(states[0]),
+      std::move(states[1]),
+      std::move(processed_frames)};
+
+  auto encoder_out = encoder_sess_->Run(
+      {}, encoder_input_names_ptr_.data(), encoder_inputs.data(),
+      encoder_inputs.size(), encoder_output_names_ptr_.data(),
+      encoder_output_names_ptr_.size());
+
+  std::vector<Ort::Value> next_states;
+  next_states.reserve(2);
+  next_states.push_back(std::move(encoder_out[1]));
+  next_states.push_back(std::move(encoder_out[2]));
+
+  return {std::move(encoder_out[0]), std::move(next_states)};
+}
+
+Ort::Value OnlineConformerTransducerModel::RunDecoder(
+    Ort::Value decoder_input) {
+  auto decoder_out = decoder_sess_->Run(
+      {}, decoder_input_names_ptr_.data(), &decoder_input, 1,
+      decoder_output_names_ptr_.data(), decoder_output_names_ptr_.size());
+  return std::move(decoder_out[0]);
+}
+
+Ort::Value OnlineConformerTransducerModel::RunJoiner(Ort::Value encoder_out,
+                                                     Ort::Value decoder_out) {
+  std::array<Ort::Value, 2> joiner_input = {std::move(encoder_out),
+                                            std::move(decoder_out)};
+  auto logit =
+      joiner_sess_->Run({}, joiner_input_names_ptr_.data(), joiner_input.data(),
+                        joiner_input.size(), joiner_output_names_ptr_.data(),
+                        joiner_output_names_ptr_.size());
+
+  return std::move(logit[0]);
+}
+
+}  // namespace sherpa_onnx
diff --git a/sherpa-onnx/csrc/online-conformer-transducer-model.h b/sherpa-onnx/csrc/online-conformer-transducer-model.h
new file mode 100644
index 00000000..f60ed53c
--- /dev/null
+++ b/sherpa-onnx/csrc/online-conformer-transducer-model.h
@@ -0,0 +1,108 @@
+// sherpa-onnx/csrc/online-conformer-transducer-model.h
+//
+// Copyright (c) 2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#ifndef SHERPA_ONNX_CSRC_ONLINE_CONFORMER_TRANSDUCER_MODEL_H_
+#define SHERPA_ONNX_CSRC_ONLINE_CONFORMER_TRANSDUCER_MODEL_H_
+
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#if __ANDROID_API__ >= 9
+#include "android/asset_manager.h"
+#include "android/asset_manager_jni.h"
+#endif
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+#include "sherpa-onnx/csrc/online-transducer-model-config.h"
+#include "sherpa-onnx/csrc/online-transducer-model.h"
+
+namespace sherpa_onnx {
+
+class OnlineConformerTransducerModel : public OnlineTransducerModel {
+ public:
+  explicit OnlineConformerTransducerModel(
+      const OnlineTransducerModelConfig &config);
+
+#if __ANDROID_API__ >= 9
+  OnlineConformerTransducerModel(AAssetManager *mgr,
+                                 const OnlineTransducerModelConfig &config);
+#endif
+
+  std::vector<Ort::Value> StackStates(
+      const std::vector<std::vector<Ort::Value>> &states) const override;
+
+  std::vector<std::vector<Ort::Value>> UnStackStates(
+      const std::vector<Ort::Value> &states) const override;
+
+  std::vector<Ort::Value> GetEncoderInitStates() override;
+
+  std::pair<Ort::Value, std::vector<Ort::Value>> RunEncoder(
+      Ort::Value features, std::vector<Ort::Value> states,
+      Ort::Value processed_frames) override;
+
+  Ort::Value RunDecoder(Ort::Value decoder_input) override;
+
+  Ort::Value RunJoiner(Ort::Value encoder_out, Ort::Value decoder_out) override;
+
+  int32_t ContextSize() const override { return context_size_; }
+
+  int32_t ChunkSize() const override { return T_; }
+
+  int32_t ChunkShift() const override { return decode_chunk_len_; }
+
+  int32_t VocabSize() const override { return vocab_size_; }
+  OrtAllocator *Allocator() override { return allocator_; }
+
+ private:
+  void InitEncoder(void *model_data, size_t model_data_length);
+  void InitDecoder(void *model_data, size_t model_data_length);
+  void InitJoiner(void *model_data, size_t model_data_length);
+
+ private:
+  Ort::Env env_;
+  Ort::SessionOptions sess_opts_;
+  Ort::AllocatorWithDefaultOptions allocator_;
+
+  std::unique_ptr<Ort::Session> encoder_sess_;
+  std::unique_ptr<Ort::Session> decoder_sess_;
+  std::unique_ptr<Ort::Session> joiner_sess_;
+
+  std::vector<std::string> encoder_input_names_;
+  std::vector<const char *> encoder_input_names_ptr_;
+
+  std::vector<std::string> encoder_output_names_;
+  std::vector<const char *> encoder_output_names_ptr_;
+
+  std::vector<std::string> decoder_input_names_;
+  std::vector<const char *> decoder_input_names_ptr_;
+
+  std::vector<std::string> decoder_output_names_;
+  std::vector<const char *> decoder_output_names_ptr_;
+
+  std::vector<std::string> joiner_input_names_;
+  std::vector<const char *> joiner_input_names_ptr_;
+
+  std::vector<std::string> joiner_output_names_;
+  std::vector<const char *> joiner_output_names_ptr_;
+
+  OnlineTransducerModelConfig config_;
+
+  int32_t num_encoder_layers_ = 0;
+  int32_t T_ = 0;
+  int32_t decode_chunk_len_ = 0;
+  int32_t cnn_module_kernel_ = 0;
+  int32_t context_size_ = 0;
+  int32_t left_context_ = 0;
+  // TODO(jingzhaoou): to retrieve from model medadata
+  int32_t right_context_ = 4;
+  int32_t encoder_dim_ = 0;
+  int32_t pad_length_ = 0;
+  int32_t vocab_size_ = 0;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_ONLINE_CONFORMER_TRANSDUCER_MODEL_H_
diff --git a/sherpa-onnx/csrc/online-lstm-transducer-model.cc b/sherpa-onnx/csrc/online-lstm-transducer-model.cc
index 25e300ae..ee40b10c 100644
--- a/sherpa-onnx/csrc/online-lstm-transducer-model.cc
+++ b/sherpa-onnx/csrc/online-lstm-transducer-model.cc
@@ -227,7 +227,8 @@ std::vector<Ort::Value> OnlineLstmTransducerModel::GetEncoderInitStates() {
 
 std::pair<Ort::Value, std::vector<Ort::Value>>
 OnlineLstmTransducerModel::RunEncoder(Ort::Value features,
-                                      std::vector<Ort::Value> states) {
+                                      std::vector<Ort::Value> states,
+                                      Ort::Value /* processed_frames */) {
   std::array<Ort::Value, 3> encoder_inputs = {
       std::move(features), std::move(states[0]), std::move(states[1])};
 
diff --git a/sherpa-onnx/csrc/online-lstm-transducer-model.h b/sherpa-onnx/csrc/online-lstm-transducer-model.h
index 5b6ad282..ab673a19 100644
--- a/sherpa-onnx/csrc/online-lstm-transducer-model.h
+++ b/sherpa-onnx/csrc/online-lstm-transducer-model.h
@@ -38,7 +38,8 @@ class OnlineLstmTransducerModel : public OnlineTransducerModel {
   std::vector<Ort::Value> GetEncoderInitStates() override;
 
   std::pair<Ort::Value, std::vector<Ort::Value>> RunEncoder(
-      Ort::Value features, std::vector<Ort::Value> states) override;
+      Ort::Value features, std::vector<Ort::Value> states,
+      Ort::Value processed_frames) override;
 
   Ort::Value RunDecoder(Ort::Value decoder_input) override;
 
diff --git a/sherpa-onnx/csrc/online-recognizer.cc b/sherpa-onnx/csrc/online-recognizer.cc
index 0cd68653..b1126cd3 100644
--- a/sherpa-onnx/csrc/online-recognizer.cc
+++ b/sherpa-onnx/csrc/online-recognizer.cc
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <iomanip>
+#include <iostream>
 #include <memory>
 #include <sstream>
 #include <utility>
@@ -187,11 +188,14 @@ class OnlineRecognizer::Impl {
     std::vector<OnlineTransducerDecoderResult> results(n);
     std::vector<float> features_vec(n * chunk_size * feature_dim);
     std::vector<std::vector<Ort::Value>> states_vec(n);
+    std::vector<int64_t> all_processed_frames(n);
 
     for (int32_t i = 0; i != n; ++i) {
+      const auto num_processed_frames = ss[i]->GetNumProcessedFrames();
       std::vector<float> features =
-          ss[i]->GetFrames(ss[i]->GetNumProcessedFrames(), chunk_size);
+          ss[i]->GetFrames(num_processed_frames, chunk_size);
 
+      // Question: should num_processed_frames include chunk_shift?
       ss[i]->GetNumProcessedFrames() += chunk_shift;
 
       std::copy(features.begin(), features.end(),
@@ -199,6 +203,7 @@ class OnlineRecognizer::Impl {
 
       results[i] = std::move(ss[i]->GetResult());
       states_vec[i] = std::move(ss[i]->GetStates());
+      all_processed_frames[i] = num_processed_frames;
     }
 
     auto memory_info =
@@ -210,9 +215,20 @@ class OnlineRecognizer::Impl {
                                             features_vec.size(), x_shape.data(),
                                             x_shape.size());
 
+    std::array<int64_t, 1> processed_frames_shape{
+      static_cast<int64_t>(all_processed_frames.size())};
+
+    Ort::Value processed_frames = Ort::Value::CreateTensor(
+      memory_info,
+      all_processed_frames.data(),
+      all_processed_frames.size(),
+      processed_frames_shape.data(),
+      processed_frames_shape.size());
+
     auto states = model_->StackStates(states_vec);
 
-    auto pair = model_->RunEncoder(std::move(x), std::move(states));
+    auto pair = model_->RunEncoder(
+      std::move(x), std::move(states), std::move(processed_frames));
 
     decoder_->Decode(std::move(pair.first), &results);
 
diff --git a/sherpa-onnx/csrc/online-transducer-model.cc b/sherpa-onnx/csrc/online-transducer-model.cc
index 89ad630e..75fb3c56 100644
--- a/sherpa-onnx/csrc/online-transducer-model.cc
+++ b/sherpa-onnx/csrc/online-transducer-model.cc
@@ -10,11 +10,13 @@
 #endif
 
 #include <algorithm>
+#include <iostream>
 #include <memory>
 #include <sstream>
 #include <string>
 
 #include "sherpa-onnx/csrc/macros.h"
+#include "sherpa-onnx/csrc/online-conformer-transducer-model.h"
 #include "sherpa-onnx/csrc/online-lstm-transducer-model.h"
 #include "sherpa-onnx/csrc/online-zipformer-transducer-model.h"
 #include "sherpa-onnx/csrc/onnx-utils.h"
@@ -22,6 +24,7 @@
 namespace {
 
 enum class ModelType {
+  kConformer,
   kLstm,
   kZipformer,
   kUnkown,
@@ -57,7 +60,9 @@ static ModelType GetModelType(char *model_data, size_t model_data_length,
     return ModelType::kUnkown;
   }
 
-  if (model_type.get() == std::string("lstm")) {
+  if (model_type.get() == std::string("conformer")) {
+    return ModelType::kConformer;
+  } else if (model_type.get() == std::string("lstm")) {
     return ModelType::kLstm;
   } else if (model_type.get() == std::string("zipformer")) {
     return ModelType::kZipformer;
@@ -78,6 +83,8 @@ std::unique_ptr<OnlineTransducerModel> OnlineTransducerModel::Create(
   }
 
   switch (model_type) {
+    case ModelType::kConformer:
+      return std::make_unique<OnlineConformerTransducerModel>(config);
     case ModelType::kLstm:
       return std::make_unique<OnlineLstmTransducerModel>(config);
     case ModelType::kZipformer:
@@ -132,6 +139,8 @@ std::unique_ptr<OnlineTransducerModel> OnlineTransducerModel::Create(
   auto model_type = GetModelType(buffer.data(), buffer.size(), config.debug);
 
   switch (model_type) {
+    case ModelType::kConformer:
+      return std::make_unique<OnlineConformerTransducerModel>(mgr, config);
     case ModelType::kLstm:
       return std::make_unique<OnlineLstmTransducerModel>(mgr, config);
     case ModelType::kZipformer:
diff --git a/sherpa-onnx/csrc/online-transducer-model.h b/sherpa-onnx/csrc/online-transducer-model.h
index 3a034516..42539de9 100644
--- a/sherpa-onnx/csrc/online-transducer-model.h
+++ b/sherpa-onnx/csrc/online-transducer-model.h
@@ -64,6 +64,7 @@ class OnlineTransducerModel {
    *
    * @param features  A tensor of shape (N, T, C). It is changed in-place.
    * @param states  Encoder state of the previous chunk. It is changed in-place.
+   * @param processed_frames  Processed frames before subsampling. It is a 1-D tensor with data type int64_t.
    *
    * @return Return a tuple containing:
    *           - encoder_out, a tensor of shape (N, T', encoder_out_dim)
@@ -71,7 +72,8 @@ class OnlineTransducerModel {
    */
   virtual std::pair<Ort::Value, std::vector<Ort::Value>> RunEncoder(
       Ort::Value features,
-      std::vector<Ort::Value> states) = 0;  // NOLINT
+      std::vector<Ort::Value> states,
+      Ort::Value processed_frames) = 0;  // NOLINT
 
   /** Run the decoder network.
    *
diff --git a/sherpa-onnx/csrc/online-zipformer-transducer-model.cc b/sherpa-onnx/csrc/online-zipformer-transducer-model.cc
index 4265a76f..7af95cc9 100644
--- a/sherpa-onnx/csrc/online-zipformer-transducer-model.cc
+++ b/sherpa-onnx/csrc/online-zipformer-transducer-model.cc
@@ -434,7 +434,8 @@ std::vector<Ort::Value> OnlineZipformerTransducerModel::GetEncoderInitStates() {
 
 std::pair<Ort::Value, std::vector<Ort::Value>>
 OnlineZipformerTransducerModel::RunEncoder(Ort::Value features,
-                                           std::vector<Ort::Value> states) {
+                                           std::vector<Ort::Value> states,
+                                           Ort::Value /* processed_frames */) {
   std::vector<Ort::Value> encoder_inputs;
   encoder_inputs.reserve(1 + states.size());
 
diff --git a/sherpa-onnx/csrc/online-zipformer-transducer-model.h b/sherpa-onnx/csrc/online-zipformer-transducer-model.h
index c2f237a3..b3e1966a 100644
--- a/sherpa-onnx/csrc/online-zipformer-transducer-model.h
+++ b/sherpa-onnx/csrc/online-zipformer-transducer-model.h
@@ -39,7 +39,8 @@ class OnlineZipformerTransducerModel : public OnlineTransducerModel {
   std::vector<Ort::Value> GetEncoderInitStates() override;
 
   std::pair<Ort::Value, std::vector<Ort::Value>> RunEncoder(
-      Ort::Value features, std::vector<Ort::Value> states) override;
+      Ort::Value features, std::vector<Ort::Value> states,
+      Ort::Value processed_frames) override;
 
   Ort::Value RunDecoder(Ort::Value decoder_input) override;
 
diff --git a/sherpa-onnx/csrc/onnx-utils.cc b/sherpa-onnx/csrc/onnx-utils.cc
index 99ca4416..80f01e49 100644
--- a/sherpa-onnx/csrc/onnx-utils.cc
+++ b/sherpa-onnx/csrc/onnx-utils.cc
@@ -168,6 +168,26 @@ void Print3D(Ort::Value *v) {
   fprintf(stderr, "\n");
 }
 
+void Print4D(Ort::Value *v) {
+  std::vector<int64_t> shape = v->GetTensorTypeAndShapeInfo().GetShape();
+  const float *d = v->GetTensorData<float>();
+
+  for (int32_t p = 0; p != static_cast<int32_t>(shape[0]); ++p) {
+    fprintf(stderr, "---plane %d---\n", p);
+    for (int32_t q = 0; q != static_cast<int32_t>(shape[1]); ++q) {
+      fprintf(stderr, "---subplane %d---\n", q);
+      for (int32_t r = 0; r != static_cast<int32_t>(shape[2]); ++r) {
+        for (int32_t c = 0; c != static_cast<int32_t>(shape[3]); ++c, ++d) {
+          fprintf(stderr, "%.3f ", *d);
+        }
+        fprintf(stderr, "\n");
+      }
+      fprintf(stderr, "\n");
+    }
+  }
+  fprintf(stderr, "\n");
+}
+
 std::vector<char> ReadFile(const std::string &filename) {
   std::ifstream input(filename, std::ios::binary);
   std::vector<char> buffer(std::istreambuf_iterator<char>(input), {});
diff --git a/sherpa-onnx/csrc/onnx-utils.h b/sherpa-onnx/csrc/onnx-utils.h
index 34ebc92e..3dc0e0fc 100644
--- a/sherpa-onnx/csrc/onnx-utils.h
+++ b/sherpa-onnx/csrc/onnx-utils.h
@@ -75,6 +75,9 @@ void Print2D(Ort::Value *v);
 // Print a 3-D tensor to stderr
 void Print3D(Ort::Value *v);
 
+// Print a 4-D tensor to stderr
+void Print4D(Ort::Value *v);
+
 template <typename T = float>
 void Fill(Ort::Value *tensor, T value) {
   auto n = tensor->GetTypeInfo().GetTensorTypeAndShapeInfo().GetElementCount();
diff --git a/sherpa-onnx/csrc/stack-test.cc b/sherpa-onnx/csrc/stack-test.cc
new file mode 100644
index 00000000..45a8dfaa
--- /dev/null
+++ b/sherpa-onnx/csrc/stack-test.cc
@@ -0,0 +1,254 @@
+// sherpa-onnx/csrc/stack-test.cc
+//
+// Copyright (c) 2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#include "sherpa-onnx/csrc/stack.h"
+
+#include "gtest/gtest.h"
+#include "sherpa-onnx/csrc/onnx-utils.h"
+
+namespace sherpa_onnx {
+
+TEST(Stack, Test1DTensors) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 1> a_shape{3};
+  std::array<int64_t, 1> b_shape{3};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(allocator, a_shape.data(),
+    a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(allocator, b_shape.data(),
+    b_shape.size());
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0; i != static_cast<int32_t>(b_shape[0]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 0);
+
+  Print1D(&a);
+  Print1D(&b);
+  Print2D(&ans);
+
+  const float *pans = ans.GetTensorData<float>();
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0]); ++i) {
+    EXPECT_EQ(pa[i], pans[i]);
+  }
+
+  for (int32_t i = 0; i != static_cast<int32_t>(b_shape[0]); ++i) {
+    EXPECT_EQ(pb[i], pans[i + a_shape[0]]);
+  }
+}
+
+TEST(Stack, Test2DTensorsDim0) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 2> a_shape{2, 3};
+  std::array<int64_t, 2> b_shape{2, 3};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(
+    allocator, a_shape.data(), a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(
+    allocator, b_shape.data(), b_shape.size());
+
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0] * a_shape[1]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0; i != static_cast<int32_t>(b_shape[0] * b_shape[1]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 0);
+
+  Print2D(&a);
+  Print2D(&b);
+  Print3D(&ans);
+
+  const float *pans = ans.GetTensorData<float>();
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0] * a_shape[1]); ++i) {
+    EXPECT_EQ(pa[i], pans[i]);
+  }
+  for (int32_t i = 0; i != static_cast<int32_t>(b_shape[0] * b_shape[1]); ++i) {
+    EXPECT_EQ(pb[i], pans[i + a_shape[0] * a_shape[1]]);
+  }
+}
+
+TEST(Stack, Test2DTensorsDim1) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 2> a_shape{4, 3};
+  std::array<int64_t, 2> b_shape{4, 3};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(allocator, a_shape.data(),
+                                                 a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(allocator, b_shape.data(),
+                                                 b_shape.size());
+
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0] * a_shape[1]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0; i != static_cast<int32_t>(b_shape[0] * b_shape[1]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 1);
+
+  Print2D(&a);
+  Print2D(&b);
+  Print3D(&ans);
+
+  const float *pans = ans.GetTensorData<float>();
+
+  for (int32_t r = 0; r != static_cast<int32_t>(a_shape[0]); ++r) {
+    for (int32_t i = 0; i != static_cast<int32_t>(a_shape[1]);
+         ++i, ++pa, ++pans) {
+      EXPECT_EQ(*pa, *pans);
+    }
+
+    for (int32_t i = 0; i != static_cast<int32_t>(b_shape[1]);
+         ++i, ++pb, ++pans) {
+      EXPECT_EQ(*pb, *pans);
+    }
+  }
+}
+
+TEST(Stack, Test3DTensorsDim0) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 3> a_shape{2, 3, 2};
+  std::array<int64_t, 3> b_shape{2, 3, 2};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(allocator, a_shape.data(),
+                                                 a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(allocator, b_shape.data(),
+                                                 b_shape.size());
+
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(a_shape[0] * a_shape[1] * a_shape[2]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(b_shape[0] * b_shape[1] * b_shape[2]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 0);
+
+  const float *pans = ans.GetTensorData<float>();
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(a_shape[0] * a_shape[1] * a_shape[2]); ++i) {
+    EXPECT_EQ(pa[i], pans[i]);
+  }
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(b_shape[0] * b_shape[1] * b_shape[2]); ++i) {
+    EXPECT_EQ(pb[i], pans[i + a_shape[0] * a_shape[1] * a_shape[2]]);
+  }
+
+  Print3D(&a);
+  Print3D(&b);
+  Print4D(&ans);
+}
+
+TEST(Stack, Test3DTensorsDim1) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 3> a_shape{2, 2, 3};
+  std::array<int64_t, 3> b_shape{2, 2, 3};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(allocator, a_shape.data(),
+                                                 a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(allocator, b_shape.data(),
+                                                 b_shape.size());
+
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(a_shape[0] * a_shape[1] * a_shape[2]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(b_shape[0] * b_shape[1] * b_shape[2]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 1);
+
+  const float *pans = ans.GetTensorData<float>();
+
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0]); ++i) {
+    for (int32_t k = 0; k != static_cast<int32_t>(a_shape[1] * a_shape[2]);
+         ++k, ++pa, ++pans) {
+      EXPECT_EQ(*pa, *pans);
+    }
+
+    for (int32_t k = 0; k != static_cast<int32_t>(b_shape[1] * b_shape[2]);
+         ++k, ++pb, ++pans) {
+      EXPECT_EQ(*pb, *pans);
+    }
+  }
+
+  Print3D(&a);
+  Print3D(&b);
+  Print4D(&ans);
+}
+
+TEST(Stack, Test3DTensorsDim2) {
+  Ort::AllocatorWithDefaultOptions allocator;
+
+  std::array<int64_t, 3> a_shape{2, 3, 4};
+  std::array<int64_t, 3> b_shape{2, 3, 4};
+
+  Ort::Value a = Ort::Value::CreateTensor<float>(allocator, a_shape.data(),
+                                                 a_shape.size());
+
+  Ort::Value b = Ort::Value::CreateTensor<float>(allocator, b_shape.data(),
+                                                 b_shape.size());
+
+  float *pa = a.GetTensorMutableData<float>();
+  float *pb = b.GetTensorMutableData<float>();
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(a_shape[0] * a_shape[1] * a_shape[2]); ++i) {
+    pa[i] = i;
+  }
+  for (int32_t i = 0;
+       i != static_cast<int32_t>(b_shape[0] * b_shape[1] * b_shape[2]); ++i) {
+    pb[i] = i + 10;
+  }
+
+  Ort::Value ans = Stack(allocator, {&a, &b}, 2);
+
+  const float *pans = ans.GetTensorData<float>();
+
+  for (int32_t i = 0; i != static_cast<int32_t>(a_shape[0] * a_shape[1]); ++i) {
+    for (int32_t k = 0; k != static_cast<int32_t>(a_shape[2]);
+         ++k, ++pa, ++pans) {
+      EXPECT_EQ(*pa, *pans);
+    }
+
+    for (int32_t k = 0; k != static_cast<int32_t>(b_shape[2]);
+         ++k, ++pb, ++pans) {
+      EXPECT_EQ(*pb, *pans);
+    }
+  }
+
+  Print3D(&a);
+  Print3D(&b);
+  Print4D(&ans);
+}
+
+}  // namespace sherpa_onnx
diff --git a/sherpa-onnx/csrc/stack.cc b/sherpa-onnx/csrc/stack.cc
new file mode 100644
index 00000000..c7ae6bee
--- /dev/null
+++ b/sherpa-onnx/csrc/stack.cc
@@ -0,0 +1,101 @@
+// sherpa-onnx/csrc/stack.cc
+//
+// Copyright (c) 2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#include "sherpa-onnx/csrc/stack.h"
+
+#include <algorithm>
+#include <functional>
+#include <iostream>
+#include <numeric>
+#include <utility>
+
+#include "sherpa-onnx/csrc/onnx-utils.h"
+
+namespace sherpa_onnx {
+
+static bool Compare(const std::vector<int64_t> &a,
+                    const std::vector<int64_t> &b) {
+  if (a.size() != b.size()) return false;
+
+  for (int32_t i = 0; i != static_cast<int32_t>(a.size()); ++i) {
+    if (a[i] != b[i]) return false;
+  }
+
+  return true;
+}
+
+static void PrintShape(const std::vector<int64_t> &a) {
+  for (auto i : a) {
+    fprintf(stderr, "%d ", static_cast<int32_t>(i));
+  }
+  fprintf(stderr, "\n");
+}
+
+template <typename T /*=float*/>
+Ort::Value Stack(OrtAllocator *allocator,
+                 const std::vector<const Ort::Value *> &values, int32_t dim) {
+  std::vector<int64_t> v0_shape =
+    values[0]->GetTensorTypeAndShapeInfo().GetShape();
+
+  for (int32_t i = 1; i != static_cast<int32_t>(values.size()); ++i) {
+    auto s = values[i]->GetTensorTypeAndShapeInfo().GetShape();
+    bool ret = Compare(v0_shape, s);
+    if (!ret) {
+      fprintf(stderr, "Incorrect shape in Stack !\n");
+
+      fprintf(stderr, "Shape for tensor 0: ");
+      PrintShape(v0_shape);
+
+      fprintf(stderr, "Shape for tensor %d: ", i);
+      PrintShape(s);
+
+      exit(-1);
+    }
+  }
+
+  std::vector<int64_t> ans_shape;
+  ans_shape.reserve(v0_shape.size() + 1);
+  ans_shape.insert(ans_shape.end(), v0_shape.data(), v0_shape.data() + dim);
+  ans_shape.push_back(values.size());
+  ans_shape.insert(
+    ans_shape.end(),
+    v0_shape.data() + dim,
+    v0_shape.data() + v0_shape.size());
+
+  auto leading_size = static_cast<int32_t>(std::accumulate(
+    v0_shape.begin(), v0_shape.begin() + dim, 1, std::multiplies<int64_t>()));
+
+  auto trailing_size = static_cast<int32_t>(
+    std::accumulate(v0_shape.begin() + dim,
+                    v0_shape.end(), 1,
+                    std::multiplies<int64_t>()));
+
+  Ort::Value ans = Ort::Value::CreateTensor<T>(
+    allocator, ans_shape.data(), ans_shape.size());
+  T *dst = ans.GetTensorMutableData<T>();
+
+  for (int32_t i = 0; i != leading_size; ++i) {
+    for (int32_t n = 0; n != static_cast<int32_t>(values.size()); ++n) {
+      const T *src = values[n]->GetTensorData<T>();
+      src += i * trailing_size;
+
+      std::copy(src, src + trailing_size, dst);
+      dst += trailing_size;
+    }
+  }
+
+  return ans;
+}
+
+template Ort::Value Stack<float>(
+  OrtAllocator *allocator,
+  const std::vector<const Ort::Value *> &values,
+  int32_t dim);
+
+template Ort::Value Stack<int64_t>(
+  OrtAllocator *allocator,
+  const std::vector<const Ort::Value *> &values,
+  int32_t dim);
+
+}  // namespace sherpa_onnx
diff --git a/sherpa-onnx/csrc/stack.h b/sherpa-onnx/csrc/stack.h
new file mode 100644
index 00000000..55f199aa
--- /dev/null
+++ b/sherpa-onnx/csrc/stack.h
@@ -0,0 +1,29 @@
+// sherpa-onnx/csrc/stack.h
+//
+// Copyright (c) 2023 Jingzhao Ou (jingzhao.ou@gmail.com)
+
+#ifndef SHERPA_ONNX_CSRC_STACK_H_
+#define SHERPA_ONNX_CSRC_STACK_H_
+
+#include <vector>
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+
+namespace sherpa_onnx {
+
+/** Stack a list of tensors along the given dim.
+ *
+ * @param allocator Allocator to allocate space for the returned tensor
+ * @param values  Pointer to a list of tensors. The shape of the tensor must
+ *                be the same except on the dim to be stacked.
+ * @param dim  The dim along which to concatenate the input tensors
+ *
+ * @return Return the stacked tensor
+ */
+template <typename T = float>
+Ort::Value Stack(OrtAllocator *allocator,
+                 const std::vector<const Ort::Value *> &values, int32_t dim);
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_STACK_H_