The aim of this study is to compare the catalytic activity of BCmNn-doped graphene sheets (Gr-BCmNn; m, n = 0, 1, 2, 3 and m + n = 3) towards the oxidation of CO molecule. By performing periodic density functional theory calculations, we show how BN co-doping can boost the surface reactivity of graphene and improve its catalytic performance in the CO + O-2 reaction. Our results indicate that the positively charged B atom in Gr-BCN2 and Gr-BN3 sheets is beneficial to the capture of O-2 molecule and serve as the catalytic active sites for the CO oxidation reaction. Both Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms are considered for the CO oxidation reaction over Gr-BCN2 and Gr-BN3. It is found that the CO oxidation proceeds first via the LH mechanism CO + O-2 -> OCOO -> CO2 + O* and then via the ER mechanism CO + O* -> CO2. The origin of high catalytic activity of these surfaces is attributed to the large electronegative difference between the B and N atoms.