In this paper, a new expression is derived for the bit error rate (BER) performance of Gray-encoded MDPSK for M = 2 and 4 in orthogonal frequency division multiplexing (OFDM) systems over time-variant and frequency-selective Rayleigh fading channels. We assume that the guard time is sufficiently Larger than the delay spread to solve the intersymbol interference (ISI) problem on the demodulated OFDM signal. In this case, the performance depends on the Doppler spread of fading channel. The closed form expression for the bit error probability of MDPSK/OFDM extended from the result in  shows that the BER performance of MDPSK is determined by (N + N-G)f(D)T(S) where N is the number of subchannels, Nc the length of the guard interval, So the maximum Doppler frequency, and T-S the sampling period. The theoretical analysis results are confirmed by computer simulations for DPSK and QDPSK signals.