We present an 8-dimensional trellis-coded 8-PSK with a symbol transition constraint that is similar to that of π/4-shift quadrature phase shift keying (QPSK). This scheme can achieve a coding gain of 1.6 to 2.4 dB at the same rate of π/4-shift QPSK on Gaussian channel, and it has also an immunity against the integer multiples of 90 phase ambiguities. In order to label the constellation of the proposed scheme, a constellation partitioning algorithm is presented. This algorithm, on the basis of set partitioning, can be used to label the signal constellation with no coset structure.