This paper describes the circuit configuration and operation mechanism of MMC. The comparisons of some modulation methods for MMC show that NLM is the best for MMC in high-power high-voltage applications， such as high voltage direct current（HVDC） transmission. The NLM method for MMC is proposed， and the formulas to calculate the fundamental and harmonic components are given. The variations of the fundamental component and the voltage THD with the number of voltage levels and the amplitude of modulation wave are exhibited. The system performance will be deteriorated in the over modulation range， which need to be avoided. Simulation results obtained with PSCAD/EMTDC are consistent with the theoretical results. Both the calculation and simulation results show that the output voltage of NLM based MMC is a good approximation to the modulation wave and has low harmonic contents in a wide operation range.