This paper is concerned with the effect of the nonlinearity of a MZM (Mach-Zehnder modulator) on optical OFDM signal distortion. The optical OFDM signal output from the MZM is expressed as a Taylor series including third-order and fifth-order nonlinear distortion terms, by which we can easily establish a mathematical model of the optical OFDM signals in different formats and rates. According to the model, we can get the EVM[1] at the optimized point. Moreover, the proposed theoretical model is proved in good agreement with experiment results of the RoF(Radio over Fiber) system.