In H.264/AVC intra frame coding, the rate-distortion optimization is employed to select the optimal coding mode to achieve the minimum rate-distortion cost. Due to a large number of combinations of coding modes, the computational burden becomes extremely high in intra prediction. In this paper, we propose a fast, efficient but reliable vector based detection algorithm for H.264 intra frame coding system. The proposed algorithm effectively estimates the edge direction inside the block to narrow down the predictive modes to reduce the RDO calculations. Experimental results show that the proposed method can reduce the encoding time by about 60% with negligible coding loss.