Nonlinearity of Semiconductor Mach-Zehnder Modulator for Flat Optical Frequency Comb Optical frequency combs generated by an InP-based semiconductor Mach-Zehnder modulator (MZM) is numerically investigated. Nonlinear phase modulation induced by drive voltages applied to the MZM prove to be useful in obtaining a flat optical frequency comb. The experimentally measured intensity deviations of optical frequency combs reported previously are quantitatively reproduced by using a numerical model taking into account nonlinear change of refractive index and optical absorption induced by the drive voltages applied to the MZM. It is found that such nonlinearities generate a flat nine-channel optical frequency comb with an intensity deviation of less than 1 dB when a moderate RF drive voltage of < 5 V is used. This flat nine-channel optical frequency comb is sustained regardless of the ratio of RF drive voltages of two arms in the MZM. Conversion efficiency is reasonably high (27%), but it is decreased by optical absorption and extinction losses in the MZM.