jian nan, it was my understanding that you need Real Player or Real One X to play these files. However, I did find
this obscure reference which indicated that the
DivX codecs for QuickTime can convert rmvb to AVI.
That doesn't sound right to me; I always thought Real's formats were proprietary. In any event, I don't have a rmvb file to test it with. You might as well install it anyway; it's free and it does allow QuickTime to play a lot of AVIs that it otherwise wouldn't be able to.
If the DivX codecs do not work, may I suggest you reconsider using the official Real client?
If you are coming from a Windows or Mac OS 9 background, I can understand why you would be wary of Real's software. However, the Mac OS X version is really a very well-behaved and elegant application; I recommend it.