Fuel cell performance and the power are influenced by factors referred to as “power loss”. In fuel cells, there are two kinds of power losses: one is dominated by the electric resistance between the electrodes, which is called as leak resistance, and the other is dominated by mass diffusion between the anode and cathode, i.e. “crossover”. In this work, we analyse the two kinds of power losses and discuss how they influence fuel cell performance. The power loss of a fuel cell caused by crossover is described by a new parameter P leak. The practical performance curve of direct methanol fuel cells using different types of membrane materials are modelled by a mathematical equation describing the power loss and crossover effect. This equation is used to estimate the methanol crossover flux.