Abstract: A mathematical model of the air electrode used in the phosphoric acid fuel cells(PAFC) was developed. This model accounts for the diffusion of oxygen and the distribution of electrode potential across the air electrode. The model was applied to simulate the cathode performance under a variety of conditions. Two parameters AB and AI were introduced into the model. They respectively reflect the effects of the interfacial surface area between the gas and liquid phases and those of the interfacial surface area between the liquid and solid phases, on the air electrode performance. AB and AI can be determined with the aid of the operating conditions and potentialcurrent density curves of the air electrode. The main interface parameters of the air electrode can be predicted after the determination of AB and AI. It is found that there is a linear relationship between the parameter AB and the current density but that parameter AI only oscillates in a very narrow range for all current densities. The reaction rate throughout the catalyst layer was calculated by means of the model, and the results show that the reduction of oxygen does not occur uniformly in the catalyst layer. The degree to which the reduction of oxygen takes place in the catalyst layer varies with current densities. The higher the operation current density, the more probability with which the reduction of oxygen takes place in the section of catalyst layer close to the diffusion layer.