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One of the computationally intensive stages in the H.264/AVC encoder is intra prediction. This is because rate-distortion optimization (RDO) is performed for the entire possible modes to select the optimum intra prediction mode. Fast intra mode decision methods have been introduced to reduce the number of tested modes by RDO to a few prediction modes. In this paper, a new fast mode decision method is introduced for H.264/AVC encoder. The proposed method is based on the idea that when one of the prediction modes achieves good RDO, its orthogonal prediction mode will not perform well. As a result, we select by simple measures only one of the orthogonal modes and perform RDO only for the selected modes. Simulation results indicate that the proposed method achieves higher peak signal-to-noise ratio (PSNR) and lower bit-rate when compared with other proposed fast mode decision methods with hardware realization. Moreover, we have proposed a three-stage pipelined architecture for our fast mode decision method, which can operate at 175 MHz maximum clock rate. Synthesis results indicate that the three-stage pipelined architecture achieves lower gate count and higher maximum clock rate when compared with hardware realizations for other fast mode decision methods and can be used to encode real-time videos of H.264/AVC standard up to level 5.1.