Electrocatalytic activity of PtNi alloy surfaces formed by sputtering was investigated toward oxygen reduction reaction (ORR) in perchloric acid solution. Hydrodynamic voltammograms with rotated electrodes were utilized to measure the activity. Maximum activity was observed at ca. 30% of Ni content by which a ten times larger kinetic current density than that of pure Pt was attained. X-ray photoelectron spectroscopy analysis of the surfaces after the reaction indicated that the active surfaces were completely covered by Pt. The effect of Pt interatomic distance at the surface or of surface roughening was excluded as the cause of high activity. We present a mechanism of the ORR based on d electron vacancy at the surface Pt due to Ni content a few monolayers below the surface. This type of catalyst will contribute in development of fuel cells for electric vehicles and other applications.