A fast intra/inter coding mode selection algorithm for the H.264 video coding standard is proposed in this paper. According to the observation of intra/inter prediction mode of any MB and those of its four neighboring blocks from different real video sequences, we find that there are a high mode correlation exists in intra/inter mode map of H.264. We exploit the interblock correlation in the intra/inter-mode domain to early terminate the rate-distortion optimization (RDO) calculations. Experimental results show that the proposed fast mode selection algorithm can yield more encoding-time saving effects with similar image quality, as compared with existing fast methods.