The activity for methanol oxidation of Pt3Ru2Sn/C was significantly higher than that of Pt/C or PtRu/C catalyst by improving CO tolerance. The higher CO tolerance property Of Pt3Ru2Sn/C was caused by the synergic effects of Ru as a water activator and Sn as an electronic modifier of Pt. In ternary alloy catalyst including the efficient water activator, the Pt-CO bond strength could become important factor.