diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index f5de29bc3cb..d2f70e7e4a8 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -188,6 +188,9 @@ Release 2.0.3-alpha - Unreleased
 
     HDFS-4377. Some trivial DN comment cleanup. (eli)
 
+    HDFS-4381. Document fsimage format details in FSImageFormat class javadoc.
+    (Jing Zhao via suresh)
+
   OPTIMIZATIONS
 
   BUG FIXES 
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
index 62d9a361478..ec6b9057740 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSImageFormat.java
@@ -48,7 +48,57 @@ import org.apache.hadoop.io.MD5Hash;
 import org.apache.hadoop.io.Text;
 
 /**
- * Contains inner classes for reading or writing the on-disk format for FSImages.
+ * Contains inner classes for reading or writing the on-disk format for
+ * FSImages.
+ * 
+ * In particular, the format of the FSImage looks like:
+ * <pre>
+ * FSImage {
+ *   LayoutVersion: int, NamespaceID: int, NumberItemsInFSDirectoryTree: long,
+ *   NamesystemGenerationStamp: long, TransactionID: long
+ *   {FSDirectoryTree, FilesUnderConstruction, SecretManagerState} (can be compressed)
+ * }
+ * 
+ * FSDirectoryTree (if {@link Feature#FSIMAGE_NAME_OPTIMIZATION} is supported) {
+ *   INodeInfo of root, NumberOfChildren of root: int
+ *   [list of INodeInfo of root's children],
+ *   [list of INodeDirectoryInfo of root's directory children]
+ * }
+ * 
+ * FSDirectoryTree (if {@link Feature#FSIMAGE_NAME_OPTIMIZATION} not supported){
+ *   [list of INodeInfo of INodes in topological order]
+ * }
+ * 
+ * INodeInfo {
+ *   {
+ *     LocalName: short + byte[]
+ *   } when {@link Feature#FSIMAGE_NAME_OPTIMIZATION} is supported
+ *   or 
+ *   {
+ *     FullPath: byte[]
+ *   } when {@link Feature#FSIMAGE_NAME_OPTIMIZATION} is not supported
+ *   ReplicationFactor: short, ModificationTime: long,
+ *   AccessTime: long, PreferredBlockSize: long,
+ *   NumberOfBlocks: int (-1 for INodeDirectory, -2 for INodeSymLink),
+ *   { 
+ *     NsQuota: long, DsQuota: long, FsPermission: short, PermissionStatus
+ *   } for INodeDirectory
+ *   or 
+ *   {
+ *     SymlinkString, FsPermission: short, PermissionStatus
+ *   } for INodeSymlink
+ *   or
+ *   {
+ *     [list of BlockInfo], FsPermission: short, PermissionStatus
+ *   } for INodeFile
+ * }
+ * 
+ * INodeDirectoryInfo {
+ *   FullPath of the directory: short + byte[],
+ *   NumberOfChildren: int, [list of INodeInfo of children INode]
+ *   [list of INodeDirectoryInfo of the directory children]
+ * }
+ * </pre>
  */
 @InterfaceAudience.Private
 @InterfaceStability.Evolving