Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has attracted great research interests over the past few years due to the immunity to fiber chromati c dispersion (CD) and polarization mode dispersion (PMD).However,CO-OFDM system is very sensitive to fiber nonli near effects,especially to four-wave mixing (FWM) effects.In CO-OFDM system,the inner-band FWM effect i s mainly dependent on serial correlation among the OFDM subcarriers.Therefore,inner-band FWM effect can b e mitigated by reducing the serial correlation among the OFDM subcarriers.In this paper,we have investigat ed and theoretically analyzed the effects of inner-band FWM in the CO-OFDM system,and then introduced a novel m ethod to reduce the serial correlation of OFDM subcarriers for mitigating the inner-band FWM effects.Our method combines the partial carrier filling (PCF) and partial sequence of transmission (PTS) together.Theore tical analysis and simulation results show that this method can effectively reduce the serial corre lation of the CO-OFDM subcarriers, and mitigate the inner-band FWM effects in CO-OFDM system.In addition,simula tion results also indicate that the bit-error-rate (BER) of the proposed system k eeps lower than 10－3 (one order of magnitude lower than PTS) after being transmitted over 1000km fiber,which demonstrates the effectiveness of the proposed method.