An equalization method based on separate frequency modified constant mod ulus algorithm (SF-MCMA) is proposed to reduce the influence of frequency selective fading (FF) caused by ch romatic dispersion in the optical orthogonal frequency division multiplexing (O-OFDM) transmission system.The th eoretical analysis shows that the reason for frequency selective fading of the optical orthogonal frequency divis ion multiplexing signal is chromatic dispersion in direct-detection O-OFDM (DD O-OFDM) transmission system without training sequence or pilot,and it is proved that the equalization based on SF-MCMA can correct the amplitude distortion and the phase deviation of the different subcarriers signals.The experimental results show that the po wer penalty of system using SF-MCMA equalization at 10e-4BER is 2dB less than that witho ut equalization when the OFDM signals transmitted over 100km standard single mode fiber (SSMF).It is proved that the method is good at dispe rsion compensation,and the system using SF-MCMA equalizer has good performance to restore signals.No training sequence or pilot sequence is required in the system,and the proposed method can save the signal bandwidth and improve the availability of the signal spectrum.