I want to reduce my video's byte,so which video format should I use with Media Player? Thanks.
M4V or MP4 would be the most normal compression containers both using H.264 encoding.
But the format isn't really the important part. You should be looking at the bitrate for the best indication of file size. For iPhone you should shoot for 1500 - 3000. Higher being bigger but better quality. iPad you'll probably need a bit more to maintain quality. Download something like Handbrake or some other format converter and you'll definitely see MP4 and H.264 encoding. Then just customize it for lower bitrate. Handbrake actually has iPhone, iPad and universal encoding presets.