OFDM-CDMA systems have been regarded as the most promising candidates for future mobile communication systems. This paper explores a novel OFDM-CDMA system with joint time-frequency spreading method proposed. The average bit error probability of this system using maximum-ratio combining (MRC) is derived in a frequency-selective fading channel. Numerical analysis and simulation results indicate in detail that the proposed system outperforms the conventional MC-CDMA system.