In order to reduce the performance impairments of directdetection optical orthogonal frequencydivision multiplexing (DDOOFDM) system caused by the nonlinear effects of electroopitc modulator and fiber chromatic dispersion, the impairment mechanisms are analyzed theoretically. Based on the analysis results, a digital preprocessing technique which combined predistortion and preemphasis methods to combat the nonlinearity and power fading, respectively, is proposed to employ to OFDM transmitter to improve the system's performance. The proposed method is verified by numerical simulation. The simulation results show that error vector magnitude (EVM) of the received signal can obtain a 2 dB improvement when both modulation indexes are optimum, compared to without employing the predistortion technique at the optical backtoback transmission case. With the increasing of optical fiber transmission distance, the power of high freqency subcarriers of the received OFDM signal is attenuated severely, then the BER performace of the system is deteriorated dramatically. The sensitivity of OFDM receiver can be improved significantly with the proposed digital preprocessing technique with low computational complexity.