The organic sol method for preparing ultrafine transition metal colloid particles reported for the first time by Bonnemann et al. [H. Bonnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Jouβen, B. Korall, Angew. Chem. Int. Ed. Engl., 30 (1991) 1312] has been improved in this paper. The improved organic sol method uses SnCl 2 as the reductant and methanol as the organic solvent. Thus, this method is very simple and inexpensive. It was found that the average size of the Pt particles in the Pt/C catalysts can be controlled by adjusting the evaporating temperature of the solvent. Therefore, the Pt/C catalysts prepared by the same method are suitable for evaluating the size effect of the Pt particles on electrocatalytic performance for methanol oxidation. The results of the X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that when the evaporating temperatures of the solvent are 65, 60, 50, 40, and 30 °C, the average sizes of the Pt particles in the Pt/C catalysts prepared are: 2.2, 3.2, 3.8, 4.3, and 4.8 nm, respectively. The X-ray photoelectron spectroscopic (XPS) results demonstrated that the small Pt particles are easily oxidized and the decomposition/adsorption of methanol cannot proceed on the surfaces of Pt oxides. Thus, the Pt/C catalyst with small Pt particles has a low electrocatalytic activity for methanol oxidation. The Pt/C catalyst with a large average size of the Pt particles also possesses a small electrochemically active surface area. Therefore, only the Pt/C catalyst with a middle average size of Pt particles, such as 3.8 nm exhibited optimal electrocatalytic performance for methanol oxidation. Because the Pt/C catalysts with the different particle sizes were prepared with the same method, the size effect on the electrocatalytic performance of the catalysts could be reliably investigated.