Abstract: H.264/AVC is the latest video compression standard enacted by ITU-T and ISO/IEC joint working group.It adopts variable block size Motion Estimation（ME） and exhaustive RDO mode decision.H.264 encoder find the best coding mode of a macroblock by computing the RDO of ten modes.So it remarkably increases the complexity of the encoder.A fast mode decision algorithm based on motion complexity is presented to improve the efficiency of the mode decision.Experimental results show that with a slight decrease of PSNR，the proposed algorithm substantially improves the encoding speed.

Key words: H.264/AVC, rate distortion, Motion Estimation（ME）, mode decision

摘要： H.264/AVC是ITU-T和ISO/IEC联合制定的最新视频压缩标准。它采用了可变块运动估计和率失真优化模式判决。H.264需要对10 种模式进行率失真优化计算才能得到一个宏块的最优编码模式，极大地增加了编码器的复杂度。为了提高模式选择效率，提出了一种基于运动复杂度的快速模式选择算法。实验结果表明，在PSNR仅有微小下降情况下，该算法可大幅提升编码速度。

关键词: H.264/AVC, 率失真, 运动估计, 模式选择