Density functional theory (DFT) calculations confirm the structural stability of isomers for 13-atom Ag, Cu, and AgCu nanoparticles. Ag-13 and Cu-13 nanoparticles have a different stable structure because of the quantum effect and differences in surface energy. We systematically studied the oxygen reduction reaction (ORR) of Ag-13, Cu-13, Ag12Cu1 (core-shell) and Ag12Cu1 (alloy) nanoparticles by investigating the adsorption property of O-2 and the transition state calculations of O-2 dissociation, which determine the ORR rate. An Ag alloy with Cu has the high adsorption energy and a low energy barrier. It also exhibits the high structural stability during the reaction. (C) 2011 Elsevier B.V. All rights reserved.