diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index e26865b877..344e24183f 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -1447,12 +1447,12 @@ static int mov_write_trun_tag(AVIOContext *pb, MOVTrack *track)
     int tr_flags=0;
     int i;
 
-    tr_flags |= 0x200; //FIXME
     for(i=track->cluster_write_index; i<track->entry; i++){
         int64_t duration = i + 1 == track->entry ?
                 track->trackDuration - track->cluster[i].dts + track->cluster[0].dts : /* readjusting */
                 track->cluster[i+1].dts - track->cluster[i].dts;
         if(duration         != 1) tr_flags |= 0x100;
+        if(track->trex_size != track->cluster[i].size) tr_flags |= 0x200;
         if(track->trex_flags != ((track->cluster[i].flags&MOV_SYNC_SAMPLE) ? 0x02000000 : 0x01010000))
                                   tr_flags |= 0x400;
         if(track->cluster[i].cts) tr_flags |= 0x800;
@@ -1913,7 +1913,8 @@ static int mov_write_trex_tag(AVIOContext *pb, MOVTrack *track, AVStream *st)
     avio_wb32(pb, track->trackID);
     avio_wb32(pb, 1); // stsd_id
     avio_wb32(pb, 1); // duration
-    avio_wb32(pb, 1/*Size*/);
+    track->trex_size= track->entry ? track->cluster[FFMIN(1, track->entry-1)].size : 1;
+    avio_wb32(pb, track->trex_size);
     track->trex_flags= st->codec->codec_type != AVMEDIA_TYPE_VIDEO ?  0x02000000 : 0x01010000;
     avio_wb32(pb, track->trex_flags);
     return updateSize(pb, pos);
diff --git a/libavformat/movenc.h b/libavformat/movenc.h
index 08dfedad8a..a3701bba0a 100644
--- a/libavformat/movenc.h
+++ b/libavformat/movenc.h
@@ -94,6 +94,7 @@ typedef struct MOVIndex {
     uint32_t    tref_tag;
     int         tref_id; ///< trackID of the referenced track
     uint32_t    trex_flags;
+    int         trex_size;
 
     int         hint_track;   ///< the track that hints this track, -1 if no hint track is set
     int         src_track;    ///< the track that this hint track describes