Electrocatalytic reduction of oxygen is one of the most important reactions for electrochemical science and technology. Although many reports have been presented on the effects of platinum particle size on the reaction, considerable discrepancies exist which seem to be caused mainly by the influence of the complex pore structure of supporting carbon materials both on the electric field of the double layer and diffusion of electrolyte and gases. This study has tried to reveal the "real size effect" by use of a carbon-supported platinum catalyst electrode model with a flat and dense carbon substrate. With a decrease in the platinum particle size, the specific activity decreases and the Tafel slope increases, the rest potential falls, and the oxygen coverage at any electrode potential in the Tafel region increases. Effects of electronic states and surface crystallography of the platinum particles on the catalytic properties have also been discussed.