A fast intra mode decision algorithm for H.264/AVC is proposed in this paper. The objective is to reduce the encoder complexity without significant rate-distortion performance degradation. Three methods are adopted: (1) the block size is selected between 4times4 block and 16times16 block based on the smoothness and correlation information. (2) A characteristic is also exploited, which is using to decide whether skip the process of mode decision for 4times4 block. (3) For other 4times4 blocks, based on the correlation and SATD information, some impossible modes are excluded. Experimental results show that the proposed algorithm can drastically reduce the encoding time with only little quality degradation.