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To-H.264 transcoding, due to its high practical values for a wide range of realtime video applications, has become an active research topic. However, most of the previous works do not address a very important new feature of H.264, multiple-reference-frame motion estimation. In this paper, employing forward domain vector selection (FDVS) and top-down splitting algorithms, a novel fast multiple-reference-frame based motion estimation & mode decision algorithm for H.263-to-H.264 transcoder is proposed. Potential block-sizes reduction and early-stop strategies are also adopted to accelerate the processing. Moreover, a novel adaptive search range selection algorithm is proposed to further speed up the motion search while the coding efficiency is not compromised. Proposed algorithm can also be extended to support transcoding other format videos to H.264, such as MPEG2-to-H.264. Experimental results show that the computational complexity can be reduced to about 11% of reference transcoder with negligible PSNR drop and a little bit-rate increase.