This paper presents a transcoder which transcodes to FGS streams from H.264/AVC hierarchical B-pictures. First, the DCT-domain architecture is designed for fast FGS transcoding. Then, we propose a mode decision method in DCT domain to achieve a trade-off between the performances at low bit-rate and high bit-rate. Experimental results demonstrated that our method can improve the coding performance up to 1 dB at low rate and only lose at worst 0.5 dB at high rate.