The oxygen reduction reaction (ORR) has been studied on the n(111)-(111) and n(111)-(100) series of Pt3Co using a hanging meniscus rotating disk electrode (HMRDE) in 0.1 M HClO4 (n is the number of terrace atomic rows). The activity for the ORR on the n(111)-(111) series, which is estimated by the specific activity at 0.90 V (RHE) j(k), increases linearly with an increase in the step atom density d(S). On the other hand, the activity for the ORR on the n(111)-(100) series increases linearly with an increase of d(S) on the surfaces with n >= 9. On the surfaces with n < 9, however, the activity for the ORR decreases with an increase of d(S). We find a correlation between the ORR activity and theta(OH) at the step on the assumption that all the oxide species are Pt-OH.