A mathematic model is established to simulate the effects of methanol crossover on the DMPC. The transport and reactions of both oxygen and methanol at the cathode are described and the theory of "parallel electrode reactions" is applied to calculate the cathode over-potential caused by methanol crossover. The influence of methanol conentration, fuel cell temperature, oxygen pressure, and membrane properties on the cathode over-potential is evaluated. Simulation results show that methanol crossover considerably increases the cathode over-potential at low current density, but its effect is significantly reduced when the current density is increased to reasonable values. It also shows that of the two parameters characterizing a polymer electrolytemembrane, proton conductivity and methanol permeability, the former has more impact on the performance of a DMPC.