We have revealed that cubelike and hexarhombic docadehedron-like Cu single crystals showed an enhancement of C2 products (ethylene and ethanol) while octahedron-like Cu nanoscale single crystals promoted C1 products of CO2 reduction. This product selectivity was revealed to be highly associated with the atomic arrangement on the surface. The remarkable high selectivity of ethanol (faraday efficiency of 25% with 14 mA/cm2, J ethanol ≈ 3.5 mA/cm2) for H–Cu was investigated and unraveled by using in situ X-ray absorption spectroscopy and density functional theory calculation. It was shown that the binding energy of adsorbed *O atom on the surface is substantial, which causes bifurcation of the reaction pathway and leads to the formation of ethanol.