diff --git a/README.md b/README.md
index 7a605aa..bd3684d 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,29 @@ ALLCING is a structured logging library featuring on lightweightness.
 - compact log data with simple binary format
 - completely-free license (WTFPLv2)
 
+# Log Data Format
+
+```
+# [TOKEN] means TOKEN is optional
+# <TOKEN> means TOKEN is constant and refer allcing.h for actual value
+
+ROOT := CHUNK_BEGIN | CHUNK_END | CHUNK_CHECK
+
+CHUNK_BEGIN := <ACG_CHUNK_BEGIN> [CHUNK_DATA]
+CHUNK_END   := <ACG_CHUNK_END>   [CHUNK_DATA]
+CHUNK_CHECK := <ACG_CHUNK_CHECK> [CHUNK_DATA]
+
+CHUNK_DATA := CHUNK_PAD | CHUNK_BLOB | CHUNK_CTX | CHUNK_LOC | CHUNK_DATA
+CHUNK_PAD  := <ACG_CHUNK_PAD>  PAD
+CHUNK_BLOB := <ACG_CHUNK_BLOB> BLOB
+CHUNK_CTX  := <ACG_CHUNK_CTX>  UINT(64) UINT(16)
+CHUNK_LOC  := <ACG_CHUNK_LOC>  UINT(8) BLOB
+
+PAD     :=
+BLOB    :=
+UINT(x) :=
+```
+
 # License
 
 Do What The Fuck You Want to Public License v2