Toward efficient CO2 electrocatalysis for CO production, nanostructured Au catalysts have been extensively investigated by the morphology control of oxygen plasma-induced Au islands, oxide-derived Au, Au nanowires (NWs), Au nanoparticles (NPs), nanoporous Au thin films, and Au needles, yet the better performance of one morphology from another is presently not well-understood, making a rational design difficult. Here, the effects of metal morphologies are investigated by focusing on Au NWs and NPs using density functional theory calculations. It is revealed that activity of two key undercoordinated active sites, namely, edge and corner sites, varies delicately with different local coordination environments of various NWs and NPs, and the observed activity trend is remarkably well-rationalized with a generalized coordination number. Furthermore, it is identified that the type of planes and the dihedral angle of the constituent planes are two key factors determining the catalytic activity. A general activity trend for CO2 reduction and H2 evolution with the consideration of the density of each type of sites explains why Au NWs exhibit better catalytic performance than Au NPs, as in experiments. On the basis of the theoretical understandings, atomic-level insights and design principles are provided toward efficiently catalyzing CO2 reduction using nanostructured metal catalysts.