Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single cryst;al electrodes examined were stepped surfaces (Pt(S)-[n(111) x (111)]), Pt(S)-[n(lll)x (100)], Pt(S)-[n(100)x (111)]), and kinked step surfaces (Pt(S)-[n(110)x (100)] and Pt(S)[n(100) x (110)]). Atomically flat surfaces, Pt(lll) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111) < Pt(100) < Pt(S)-[n(lll) x (100)] < Pt(S)-[n(lll) x (100)] < Pt(S)-[n(lll) x (111)] < Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111) x (111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2.