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Density functional theory has been applied to study the geometric and electronic structures and the catalytic properties of Ag and AgAu clusters for CO oxidation. The calculated results suggest that the doping of Au atoms improve the stability of AgAu clusters. Correspondingly, the binding energy (BE) per atom of AgnAu is larger than that of pure Agn+1 cluster, due to strong hybridization between the d orbitals of Au and the s orbitals of Ag in AgnAu clusters. With the increasing Au concentration, the BE of Ag13?nAun(n = 1–8) clusters increase smoothly, while second-order difference of energies (Δ2E) and fragmentation energies (ΔE) show an even–odd oscillation. The AgAu clusters containing an odd n umber of gold atoms (Ag13?nAun, n = 3, 5, 7) are relatively stable compared to their neighbor. The CO and O2adsorption properties on the Ag13, Ag10Au3, Ag8Au5, and Ag6Au7 clusters suggest that O2 is strongly activated by the clusters, while the activation of CO is much weak. Furthermore, the activation of O2 on AgAu cluster is stronger than that on pure Ag13 cluster, especially on Ag8Au5 cluster, due to the strengthened polarization of O–O bond. Compared to Ag13, Ag10Au3, and Ag6Au7 clusters, the lower energy barriers on Ag8Au5 cluster suggest a higher catalytic activity of Ag8Au5 cluster for O2 dissociation and CO oxidation reactions. The calculated results suggest that Ag8Au5 cluster could effectively reduce the carbon monoxide poisoning and exhibits the excellent catalytic performance for CO oxidation. Our study provides atomic-scale insights into the nature of the interfacial effects that determine CO oxidation on Ag–Au cluster catalysts.