The catalytic reduction of N2O on a single Si or P atom incorporated nitrogen-doped graphene (SiN4-Gr/PN4-Gr) is investigated by means of DFT calculations. It is found that for both surfaces, the dopant atom prefers to reside in the divacancy site. The N2O reduction by CO molecule over these surfaces proceeds via a two-step mechanism: (a) N2O -> N-2 + O*, and (b) O* + CO -> CO2. Our results indicate that the catalytic activity of SiN4-Gr is higher than that of PN4-Gr, due to the more favorable orbital interaction between CO molecule and Si atom in the corresponding transition state.