A structure and an iterative decoding algorithm of a turbo trellis-coded modulation system, proposed by Robertson and et al. , are improved. For the encoder, removal of the odd-even constraint of an interleaver is discussed and a structure which removes a serial connection of an interleaver and a deinterleaver is proposed. The latter makes encoding delay nearly half. A decoding algorithm which is a natural extension of the standard decoding algorithm to TCM is proposed. In the proposed algorithm, logarithm of an a posteriori probability ratio is divided into three component values: an a priori value, a channel value and an extrinsic information. The extrinsic information is transferred to the next decoding stage as an a priori value. The proposed algorithm is easier to understand than the Robertson's algorithm in which a combination of the channel value and the extrinsic information is transferred to the next stage. Simulation results show the proposed algorithm realizes equivalent or better performance than the Robertson's algorithm. The removal of the odd-even constraint gives a little better performance than that with odd-even constraint in some conditions. By this improvement, bit error rate of 10-5 is obtained at Eb/N0 0. 4 dB from the Shannon limit for 2 bit/symbol transmission with 8-PSK modulation.