diff --git a/core/README.md b/core/README.md
index a3603068..2b437ac4 100644
--- a/core/README.md
+++ b/core/README.md
@@ -2,6 +2,70 @@ Introduction
 ==============
 This project aims to demonstrate wasm app management and programming model of WAMR.
 
+Directory structure
+------------------------------
+<pre>
+simple/
+├── build.sh
+├── CMakeLists.txt
+├── README.md
+├── src
+│   ├── ext-lib-export.c
+│   ├── iwasm_main.c
+│   └── main.c
+└── wasm-apps
+    ├── event_publisher
+    │   └── event_publisher.c
+    ├── event_subscriber
+    │   └── event_subscriber.c
+    ├── request_handler
+    │   └── request_handler.c
+    ├── request_sender
+    │   └── request_sender.c
+    ├── sensor
+    │   └── sensor.c
+    └── timer
+        └── timer.c
+</pre>
+
+- build.sh
+  The script to build all binaries.
+- CMakeLists.txt
+  CMake file used to build the simple application.
+- README.md
+  The file you are reading currently.
+- src/ext-lib-export.c
+  This file is used to export native APIs. See README.md in WAMR root directory for detail.
+- src/iwam_main.c
+  This file should be implemented by platform integrator in which a host interface is provided to interact with WAMR app-manager. See `{WAMR_ROOT}/core/app-mgr/app-mgr-shared/app-manager-export.h` for the definition of the host interface.
+```
+  /* Interfaces of host communication */
+typedef struct host_interface {
+    host_init_func init;
+    host_send_fun send;
+    host_destroy_fun destroy;
+} host_interface;
+```
+```
+host_interface interface = {
+                             .init = host_init,
+                             .send = host_send,
+                             .destroy = host_destroy
+                           };
+```
+This interface is passed to app-manager by calling
+```
+app_manager_startup(&interface);
+```
+
+The `host_init_func` is automatically called when app-manager startup. And `host_send_fun` will be called by app-manager to send data to host anytime.
+>Note: Currently since app-manager will keep running and never exit, `host_destroy_fun` has no chance to get executed. So you can leave this API implementation empty.
+
+- src/main.c
+  The main file.
+- wasm-apps
+  Source files of sample wasm applications.
+
 Build all binaries
 ==============
 Execute the build.sh script then all binaries including wasm application files would be generated in 'out' directory.