The basic principles of dispersion are illustrated. The researching on ROF system indicates that with the increasing of transmission velocity, the dispersion will limit the performances of the transmission system, even lead to the distortion of the signal. The effects of dispersion on ROF system are summarized and analyzed from three aspects: the two different ways of amplitude modulation (DSB modulation and SSB modulation), the two different ways of modulation in the number of wavelength (solitary wavelength and dual wavelength) and the chirped phenomenon caused to the signal in its transmission. The conclusion is that SSB modulation is better than DSB modulation, which provides some theoretical basis for improving the performance of ROF system.