The development of high-performance cathode electrocatalysts in nonaqueous lithium-oxygen (Li–O2) battery is mainly restricted by high charge/discharge overpotentials. Based on first-principles calculations, we theoretically explore the application potential of BC3 and NC3 nanosheets as the suitable cathode electrocatalysts. By analyzing the stable adsorption configurations of LixO2y (x = 0–4, y = 0–2) intermediates on BC3/NC3, it is confirmed that oxygen reduction reaction (ORR) on BC3 follows four-electron pathway with (Li2O)2 as the discharge product, while for ORR on NC3, (Li2O2)2 is the discharge product following two-electron pathway. We then plot the corresponding free energy diagrams of charge/discharge processes and calculate the theoretical discharge and charge overpotentials ( η O R R and η O E R ) along the optimal reaction pathways. Our results show that both the η O R R and η O E R on BC3 are considerably low (0.21 V and 0.36 V), indicating that BC3 can serve as a high-performance cathode electrocatalyst. Meanwhile, the η O R R and η O E R on NC3 (0.37 V and 0.95 V) are comparable to previously reported materials following two-electron pathway (such as N-doped graphene), exhibiting practical values. Both the BC3 and NC3 nanosheets are stable enough for application in Li–O2 battery, and dimethyl sulfoxide (DMSO) isn't recommended as an organic electrolyte when BC3 works as the cathode catalyst.