diff --git a/ChangeLog b/ChangeLog
index c0c53cd173..098bb242bc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Nov 28 11:46:35 2011  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* io.c (rb_io_flush): release GVL during fsync() on Windows.
+
 Mon Nov 28 11:00:25 2011  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* include/ruby/subst.h: typo of r33876.
diff --git a/io.c b/io.c
index 3a0ba43097..4b5896a596 100644
--- a/io.c
+++ b/io.c
@@ -1187,6 +1187,15 @@ rb_io_addstr(VALUE io, VALUE str)
     return io;
 }
 
+#ifdef HAVE_FSYNC
+static VALUE nogvl_fsync(void *ptr)
+{
+    rb_io_t *fptr = ptr;
+
+    return (VALUE)fsync(fptr->fd);
+}
+#endif
+
 /*
  *  call-seq:
  *     ios.flush    -> ios
@@ -1220,7 +1229,7 @@ rb_io_flush(VALUE io)
             rb_sys_fail(0);
 #ifdef _WIN32
 	if (GetFileType((HANDLE)rb_w32_get_osfhandle(fptr->fd)) == FILE_TYPE_DISK) {
-	    fsync(fptr->fd);
+	    rb_thread_io_blocking_region(nogvl_fsync, fptr, fptr->fd);
 	}
 #endif
     }
@@ -1332,6 +1341,8 @@ rb_io_set_pos(VALUE io, VALUE offset)
 
 static void clear_readconv(rb_io_t *fptr);
 
+#ifdef HAVE_FSYNC
+
 /*
  *  call-seq:
  *     ios.rewind    -> 0
@@ -1508,14 +1519,6 @@ rb_io_set_sync(VALUE io, VALUE sync)
     return sync;
 }
 
-#ifdef HAVE_FSYNC
-static VALUE nogvl_fsync(void *ptr)
-{
-    rb_io_t *fptr = ptr;
-
-    return (VALUE)fsync(fptr->fd);
-}
-
 /*
  *  call-seq:
  *     ios.fsync   -> 0 or nil