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A semi-fragile content authentication algo-rithm is proposed for low bit-rate H.264/AVC video in VLC domain. Utilizing the intra prediction mode and coded block pattern in VLC domain, the pro-posed algorithm chooses those macro-blocks from which the signature is extracted and constructs content signature at macro-block level according to the relationship among the energies of quantized low-frequency coefficients of sub-macroblocks.The signature is embedded by modifying the trailing coefficients. The experimental results show that the proposed algorithm performs well in visual quality impact and keep the bit-rate basically unchanged. In addition, the algorithm can embed signatures into Ⅰ,P, B slices simultaneously and remarkably enhances the watermark capacity. By verifying the extracted signature, the algorithm can detect and locate video tampering efficiently.