The promising catalytic activity of nitrogen-doped graphene (N-graphene) for various heterogenous processes has attracted extensive attention, but the active sites of N-graphene for O-2 adsorption and the underlying activation mechanism are still unclear. Density functional theory (DFT) was used in this work to investigate the role of graphitic N (GrN), pyridinic N (PyN), and their coexistence in graphene on O-2 adsorption and activation. It was found that O-2 adsorption is energetically promoted by GrN with the adsorption site atop it. The initially protonated O-2 (OOH*) prefers to bind with the C atom adjacent to the GrN. More interesting, the combinational of PyN and GrN can further redistribute charge among the carbon materials, which subsequently promotes O-2 adsorption and lowers the energy barrier of O-2 hydrogenation, demonstrating an improved property for ORR. This finding can help to identify the active site of O-2 adsorption and protonation on N-graphene and clarify the function of different doped N, providing a guiding suggestion for the experimental design of N-doped graphene. (C) 2018 Elsevier B.V. All rights reserved.