The design of high performance cathode electrocatalysts is essential for polymer-electrolyte fuel cells, which are now attracting enormous interest as a primary power source for zero-emission electric vehicles. We have discovered a significant enhancement of electrocatalytic activity of Pt by alloying with Fe, and found a maximum activity at ca. 50% Fe content, which results in 25 times higher activity than pure Pt activity. It was confirmed experimentally at Pt-Fe bulk alloys that the alloy catalyst surface consists of a purl Pt skin-layer (<1 nm in thickness) that is modified in the electronic structure by that of the bulk alloy. The enhancement could be well explained by the Sd-vacancy of the surface, but not by Pt interatomic distance or roughening of the surfaces.