The CO oxidation reactivity of negatively and positively charged isolated cuboctahedron (c-Oh) Au-13 and Au12Ag nanoparticles is investigated using density functional theory calculations. Charging the nanoparticles modifies the structural stability of the Au-13 and Au12Ag nanoparticles as well as the electron distribution in the core and shell atoms. An Ag-doping in gold (Au) clusters improves CO or O-2 adsorption on Au12Ag cluster. For Au-13 cluster, CO preadsorption increases the capacity of CO and O-2 coadsorption, but the result is opposite for Au12Ag cluster. The neutral Au-13 and Au12Ag clusters exhibit relatively poor reactivity for CO oxidation, while the reactivity is enhanced significantly by excess electrons. In comparisons of the results of CO oxidation on Ag- and un-doped Au nanoparticles, we discover Ag-doping in Au cluster surely decreases first energy barrier (E-a), and increases slightly second energy barrier (E-b). This work provides a fundamental insight into how the excess charges affect the adsorption activity and how the Ag-doping in Au clusters adjusts the catalytic activity for Ag- or un-doped c-Oh Au clusters. Reaction pathways for CO + O-2 -> CO2 + O associated with Au-13 and Au12Ag clusters. Here, * denotes the adsorbed species on an Au-13 or Au12Ag cluster. The reactivity of CO oxidation on Au nanoparticles is enhanced significantly by excess electrons. An Ag-doping in Au cluster improves CO or O-2 adsorption on Au12Ag cluster. Ag-doping in Au clusters decreases first energy barrier (E-a), and increases slightly second energy barrier (E-b). Ag-doping in Au nanoparticles weakens C-Au bond at CO + O-2 coadsorption state, and strengths CO-O bonds at transition states and intermediate state.