Structural effects on hydrogen oxidation and evolution reactions (HOR/HER) have been studied on n(111)-(111) surfaces of Pt using a rotating disk electrode in 0.1 M HClO(4) saturated with H, at temperatures between 268 and 298 K. The exchange current density of HOR increases linearly with the increase of step atom density up to the surface with the terrace atomic rows n = 9; however, it becomes constant on the surfaces with n <= 9. The activation energy and pre-exponential factor of HOR/HER decrease with the increase of the step atom density, but they also become constant on the surfaces with n <= 9. These results support that the step atom is the active site of HOR/HER; however. a part of the step atoms contributes to HOR/HER on the surfaces with n <= 9.