Electrochemical reduction of CO2 was studied using single-crystal electrodes, Cu(111), Cu(100), Cu(S)-[n(100) x (111)], and Cu(S)-[n(100) x (110)] at a constant current density 5 mA cm(-2) in 0.1 M KHCO3 aqueous solution. Copper single crystals were prepared from 99.999% copper metal in graphite crucibles by the Bridgeman method. The crystal orientation was determined by the X-ray back reflection method. The Cu(111) electrode yields mainly CH4 from CO2, and the Cu(100) favorably gives C2H4. Introduction of (111) steps to Cu(100) basal plane, leading to Cu(S)-[n(100) x (111)] orientations, significantly promoted C2H4 formation and suppressed CH4 formation. The selectivity ratio C2H4/CH4 on Cu(711) (n = 4) amounted to 14, 2 orders of magnitude higher than that on Cu(111).