In recent years, along with the development of computer technology, compression technology and network technology, how to efficiently transmit multimedia information over network has become a main concern by industry and research institution. MPEG-4 FGS technology is introduced and the rate allocation optimizing problem during MPEG-4 FGS bitsream streaming over network is analyzed. In order to performing rate allocation, the rate-distortion function of FGS enhancement layer should be known, so this paper firstly analyzes the rate-distortion function. Secondly, several objectives that the rate allocation should meet are pointed out. Finally, two rate allocation optimizing models are proposed: constant distortion model and minimum distortion model, and the corresponding mathematic expressions are given. The result revealed that the two optimizing models are different, and they can be selected according to different network variance conditions.