The influence of Nafion thickness on electrocatalytic properties for H, oxidation and 0, reduction on Nafion-coated Pt electrodes was investigated to predict an ideal structure of catalyst layer for PEFCs. Currents for H, oxidation and 0, reduction at Nafion-coated Pt electrodes were measured as a function of thickness of the Nafion film by the rotating disk electrode method. Pt electrodes with a thin Nafion film showed catalytic activities similar to those of a bare Pt electrode. The critical thickness of the Nafion film on the catalyst surface is cu. 0.2pm, with which the diffusion process of reactant gases to catalyst sites is not the rate-determining step. With thickness more than 0.2pm, the diffusion of reactant gases in film controlled the reaction currents. On the basis of the data in this study, maximum current densities for H, oxidation and 0, reduction on the ideal catalyst layer with the critical thickness of Nafion were estimated. It appeared that there is room to improve the electrode performances by optimizing the Nafion film thickness on catalyst surfaces or by applying a new structure of catalyst layers for PEFCs.