Pt/C catalysts with different mean size of Pt particles were prepared by a microwave-assisted polyol process and characterized by TEM and XRD. The effects of the synthesis solution pH on Pt particles size and distribution, and their activity for methanol electrooxidation were investigated. Pt nanoparticles were small and uniform with mean size of 2.7 nm under the pH of 9.5. Pt/C catalyst showed high activity for methanol electrooxidation.