Abstract-This paper presents an energy-efficient turbo decoder based on border metric encoding, which is especially suitable for non-binary circular turbo codes. In the proposed method, the size of the branch memory is reduced to half and the dummy calculation is removed at the cost of a small-sized memory that holds encoded border metrics. Due to the small size and infrequent access to the border memory, power consumption for soft-input soft-output (SISO) decoding is reduced by 26.0%. Based on the proposed SISO decoder and the dedicated hardware interleaver, a double-binary tail-biting turbo decoder is designed for WiMAX standard using a 0.18 µm CMOS process and it can support 12.14Mbps at operating frequency of 100MHz.