A model catalyst method has been adopted to clarify the effect of particle size of the Pt metals supported on carbon materials that are used for the electrocatalytic oxidation of methanol in an acidic solution at 60^oC. The model catalyst electrodes, Pt/GC, were prepared by vacuum deposition of Pt on a mirror-polished surface of non-porous glassy carbon (GC) to avoid the influence of the carbon substrates pores on the catalytic properties. The electrocatalytic activity of the electrodes evaluated both by cyclic voltammetry and chronoamperometry in 0.5 M H₂SO₄ showed that the specific activity of the methanol oxidation increased with an increase in the Pt particle size. The oxidation of CO on the Pt/GC electrodes, which was examined to elucidate the size effect, indicated that smaller platinum particles need higher energy for the oxidation of carbon monoxide. The size effect on the specific activity was discussed in terms of the exposed crystal planes and characteristic valence band structure of the Pt particles supported on carbon.