i found it in fedoraforum
mencoder in.rmvb -oac mp3lame -lameopts preset=128 -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1200 -ofps 25 -of avi -o out.avi
File
1. rmvb file: in.rmvb
2. avi file: out.avi
Information for the avi file
Video
format: mpeg4
bitrate:1200 kb/s
fps: 25 fps
Audio
format: mp3
bitrate: 128 kb/s
Explanation
-oac: output audio codec
mp3lame: library used for audio encoding
-lameopts: options used along with lame
preset: values for audio bitrate, you can set 64, 128, 224, etc
-ovc: ouput video codec
lavc: library used for video encoding
-lavcopts: options used along with lavc
vcodec: video codec, you can use mpeg1video, mpeg4, etc
vbitrate: video bitrate, you can set 600, 1000, 1200, etc
-ofps: outpt frame per second ( fps)
-of: output file container type
-o: output filename