H.264, MPEG-4 Part 10, is the latest digital video coding standard that is noted for achieving very high data compression by using several new coding features. One of the key features is the intra prediction based on the rate distortion optimization (RDO) procedure, which requires a true encoding of the intra block by trying all intra prediction modes. The computational burden of intra prediction with the RDO procedure is extremely high for real applications. In this paper, we propose an efficient fast mode decision method by using bi-region similarity detection (BSD) algorithm to reduce the encoding time. The proposed BSD algorithm effectively estimates the texture direction of the block to narrow down the predictive modes to reduce RDO computation. Experimental results show that the proposed algorithm achieves more than a 60% time reduction with negligible PSNR loss and bit-rate increment.