Is there any way to LOCK xvid bitrate! ?

The best solution to your problem would be something like encoding at "constant quality" ( 80+ % ) and in addition to that define a maximum bitrate. But this isn't possible in XviD ( yet? ). As for know I suggest you use "CBR" ( which is ABR in fact ) and set all of the CBR options ( Advanced Options -> Debug ) to 0 or 1. IIRC those values represent how the codec smoothes the bitrate curve over time. If you tell the codec to only look at very short intervals and react very fast ( = low values ) it will try to make the bitrate constant. I can't garuantee for anything but it's worth a try! BTW: you shouldn't restrict quantizers too much ( 2 to 6/7/8 should be fine ) because otherwise XviD can't always remove all the details it has to to reach the bitrate you want.