diff --git a/src/sfs.rs b/src/sfs.rs
index e7eda15..9d9e471 100644
--- a/src/sfs.rs
+++ b/src/sfs.rs
@@ -271,6 +271,7 @@ impl vfs::INode for INode {
         let fs = self.fs.upgrade().unwrap();
         let info = self.info().unwrap();
         assert_eq!(info.type_, vfs::FileType::Dir);
+        assert!(info.nlinks>0);
 
         // Ensure the name is not exist
         assert!(self.get_file_inode_id(name).is_none(), "file name exist");
@@ -297,6 +298,8 @@ impl vfs::INode for INode {
         Ok(inode)
     }
     fn unlink(&mut self, name: &str) -> vfs::Result<()> {
+        assert!(name!=".");
+        assert!(name!="..");
         let fs = self.fs.upgrade().unwrap();
         let info = self.info().unwrap();
         assert_eq!(info.type_, vfs::FileType::Dir);
@@ -313,12 +316,20 @@ impl vfs::INode for INode {
             // only . and ..
             assert!(inode.borrow().disk_inode.blocks==2);
         }
+        inode.borrow_mut().nlinks_dec();
+        if(type_==FileType::Dir){
+            inode.borrow_mut().nlinks_dec();//for .
+            self.nlinks_dec();//for ..
+        }
+        let old_block_count=self.disk_inode.blocks as usize;
+        if(entry_id!=old_block_count - 1){//not last entry
+            let mut entry: DiskEntry = unsafe { uninitialized() };
+            self._read_at((old_block_count - 1) * BLKSIZE, entry.as_buf_mut()).unwrap();
+            self._write_at(entry_id * BLKSIZE, entry.as_buf()).unwrap();
+        }
+        self._resize((old_block_count - 1) * BLKSIZE).unwrap();
 
-        //todo: actually unlink:
-        //1. move the last entry to the original place
-        //2. free the disc space
-
-        unimplemented!();
+        Ok(())
     }
     fn lookup(&self, path: &str) -> vfs::Result<Ptr<vfs::INode>> {
         let fs = self.fs.upgrade().unwrap();
diff --git a/src/structs.rs b/src/structs.rs
index 2b07667..f9596f5 100644
--- a/src/structs.rs
+++ b/src/structs.rs
@@ -29,6 +29,7 @@ pub struct DiskINode {
     /// one of SYS_TYPE_* above
     pub type_: FileType,
     /// number of hard links to this file
+    /// Note: "." and ".." is counted in this nlinks
     pub nlinks: u16,
     /// number of blocks
     pub blocks: u32,
diff --git a/src/tests.rs b/src/tests.rs
index 6abffb1..a33294b 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -168,6 +168,18 @@ fn kernel_image_file_create(){
     sfs.sync().unwrap();
 }
 
+// #[test]
+fn kernel_image_file_unlink(){
+    let sfs = _open_sample_file();
+    let root = sfs.root_inode();
+    let files_count_before = root.borrow().list().unwrap().len();
+    root.borrow_mut().unlink("hello");
+    let files_count_after = root.borrow().list().unwrap().len();
+    assert_eq!(files_count_before, files_count_after+1);
+
+    sfs.sync().unwrap();
+}
+
 #[test]
 fn nlinks(){
     let sfs = _create_new_sfs();
@@ -179,6 +191,12 @@ fn nlinks(){
     let dir1 = root.borrow_mut().create("dir1", FileType::Dir).unwrap();
     assert_eq!(dir1.borrow().info().unwrap().nlinks,2);
     assert_eq!(root.borrow().info().unwrap().nlinks,3);
+    root.borrow_mut().unlink("file1").unwrap();
+    assert_eq!(file1.borrow().info().unwrap().nlinks,0);
+    assert_eq!(root.borrow().info().unwrap().nlinks,3);
+    root.borrow_mut().unlink("dir1").unwrap();
+    assert_eq!(dir1.borrow().info().unwrap().nlinks,0);
+    assert_eq!(root.borrow().info().unwrap().nlinks,2);
 
     sfs.sync().unwrap();
 }
\ No newline at end of file
diff --git a/src/vfs.rs b/src/vfs.rs
index e9b8375..1b5396c 100644
--- a/src/vfs.rs
+++ b/src/vfs.rs
@@ -37,6 +37,8 @@ pub struct FileInfo {
     pub mode: u32,
     pub type_: FileType,
     pub blocks: usize,
+    // Note: different from linux, "." and ".." count in nlinks
+    // this is same as original ucore.
     pub nlinks: usize,
 }