— Orthogonal Frequency Division Multiplexing (OFDM) is sensitive to Carrier Frequency Offset (CFO), which destroys the orthogonality and causes Inter-Carrier Interference (ICI). In this paper, we investigate the average error probability of OFDM systems impaired by CFO over frequency selective Rayleigh fading channels. By exploiting the Beaulieu series, the effects of CFO on the Symbol Error Rate (SER) and Bit Error Rate (BER) performance, which depend strongly on the statistical characteristics of the fading channels, are expressed as the sum of an infinite series in terms of the characteristic function (CHF) of the ICI. We only consider OFDM systems modulated with BPSK, QPSK, 8-PSK and 16-QAM, and the Gray-coded mapping are employed for the evaluation of BER performance.