diff --git a/lib/spdylay_session.c b/lib/spdylay_session.c
index 27ccb5f6..fa7c804e 100644
--- a/lib/spdylay_session.c
+++ b/lib/spdylay_session.c
@@ -58,7 +58,11 @@ int spdylay_outbound_item_compar(const void *lhsx, const void *rhsx)
   const spdylay_outbound_item *lhs, *rhs;
   lhs = (const spdylay_outbound_item*)lhsx;
   rhs = (const spdylay_outbound_item*)rhsx;
-  return lhs->pri-rhs->pri;
+  if(lhs->pri == rhs->pri) {
+    return (lhs->seq < rhs->seq) ? -1 : ((lhs->seq > rhs->seq) ? 1 : 0);
+  } else {
+    return lhs->pri-rhs->pri;
+  }
 }
 
 int spdylay_session_client_new(spdylay_session **session_ptr,
@@ -79,6 +83,8 @@ int spdylay_session_client_new(spdylay_session **session_ptr,
 
   (*session_ptr)->last_ping_unique_id = 0;
 
+  (*session_ptr)->next_seq = 0;
+
   (*session_ptr)->goaway_flags = SPDYLAY_GOAWAY_NONE;
   (*session_ptr)->last_good_stream_id = 0;
 
@@ -192,6 +198,7 @@ int spdylay_session_add_frame(spdylay_session *session,
   item->frame_type = frame_type;
   item->frame = frame;
   item->aux_data = aux_data;
+  item->seq = session->next_seq++;
   /* Set priority lowest at the moment. */
   item->pri = 3;
   switch(frame_type) {
diff --git a/lib/spdylay_session.h b/lib/spdylay_session.h
index fea9e3a1..cc766dc7 100644
--- a/lib/spdylay_session.h
+++ b/lib/spdylay_session.h
@@ -41,6 +41,7 @@ typedef struct {
   spdylay_frame *frame;
   void *aux_data;
   int pri;
+  int64_t seq;
 } spdylay_outbound_item;
 
 typedef struct {
@@ -94,6 +95,10 @@ struct spdylay_session {
      SPDYLAY_MAX_UNIQUE_ID */
   uint32_t next_unique_id;
 
+  /* Sequence number of outbound frame to maintain the order of
+     enqueue if priority is equal. */
+  int64_t next_seq;
+
   spdylay_map /* <spdylay_stream*> */ streams;
   spdylay_pq /* <spdylay_outbound_item*> */ ob_pq;