MPEG－4作为一种适合各种多媒体应用的图像压缩编码标准，已经得到广泛应用。介绍了I帧、P帧、B帧的运动图像编码。论述了影响图像恢复质量的几个因素，包括以下几个方面：DCT(Discrete cosine Transform)离散余弦变换；QP(Quantize Parameter)量化系数；SR(Search Range)运动估计中搜索范围以及半像素搜索(half－pixel searching)等。通过压缩比，峰值信噪比，编解码时间等大量实验数据比较各因素影响大小。实验证明量化系数对图形恢复质量影响最大，其余几个因素也有一定影响。

MPEG-4 has been the most popular image compression standard. The process of I frame,P frame and B frames'coding based on MPEG-4 standard was presented . Some factors that affect the compressed quality were described,DCT trans-form,quantity parameter,searching range and half-pixel searching in the motion estimation. Many experiment data including compression ratio,PSNR and codec time etc were compared for the influence of these parameters,in which QP is the most significant parameter and the others have certain effect.