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H.264/AVC is the latest international video coding standard, which provides a superior coding efficiency, compared with MPEG-4 and other previous standards. Context Adaptive Binary Arithmetic Coding (CABAC) was adopted by H.264/AVC and plays a very important role in improving the video coding efficiency. However, the coding speed of CABAC is restricted by high computational complexity, which becomes a major bottleneck in many applications. According to the analysis of the CABAC coding flow and its computational complexity, an effective accelerating CABAC algorithm based on adaptive probability estimation update (APU-CABAC) to improve its coding speed is proposed. Instead of every binary symbol's coding, the probability estimation update part in APU-CABAC is done after every packet. In addition, the packet size is adaptively adjusted according to the comparison of the symbol changing frequency coefficient of the last and current packets. Experimental results show that APU-CABAC provides a substantial improvement in the coding speed of CABAC while coding efficiency declines only a little.