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In this paper, we propose an efficient transform domain method for transcoding an intra frame of H.264/AVC to a JPEG image. First, we show how to obtain the transform domain predictions for various intra modes of H.264/AVC through simplified matrix multiplication. Then, we describe a transform domain operation corresponding to the rounding operation which is performed after computing the weighted average during spatial domain intra prediction. Finally, we convert 4 × 4 integer transform blocks to 8 × 8 DCT blocks by defining a transform kernel matrix based on the factorization of transform matrices. Experimental results demonstrate that the proposed algorithm can reduce the total computational complexity about 16% on average as compared with the existing methods, and outperforms them in PSNR by 1.5~2.5dB.