For an additive white Gaussian noise (AWGN) channel, a performance evaluation of parallel concatenated turbo trellis-coded modulation (turbo TCM) using bit-interleavers is reported. By obtaining weight distribution, the performance is evaluated by using a union bound method. Comparison between the result of evaluated performance and simulation results is shown, and the usefulness of the evaluated performance is shown. An optimum code and an optimum mapping are sought. The result of the optimum code with the optimum mapping is a new interleaver size N dependency which is proportional to N-3. It is better than the interleaver size dependency for Benedetto code with the natural mapping which is proportional to N-1. The reasons why these dependencies can happen are also discussed.