The origin of enhanced electrocatalytic activity of Pt–M (M=Fe, Co, Ni, Cu, and W) alloy catalysts in PEM fuel cell cathodes have been investigated by the periodic DFT geometry optimization and electronic structure calculations. The stability of Pt–M alloy catalysts and the electrocatalytic activity have been explained, and we confirmed that the electronic structure of surface Pt is tuned after alloying Pt with non-noble transition metals, thus leading to enhancement of the electrocatalytic activity of Pt–M alloys for the ORR. Meanwhile, a positive shift in reversible potential was predicted for OH ads formation from H 2 O oxidation on the Pt 3 M alloy catalysts with Pt-skin compared to the pure Pt. Therefore, this improvement of electrocatalytic activity of the ORR can be also attributed to a positive shift of the onset potential for forming OH ads on the alloys relative to the Pt catalyst; thereby the overpotential for O 2 reduction is reduced. In these ways we could screen new electrocatalysts for the ORR.