The first-principle calculations are performed to explore the synergy effects between two dopants (N, S) and the detailed reaction pathway of oxygen reduction reaction (ORR) on the graphene catalysts. The co-doping N and S induces the significant spin density and has a strong chemical bonding with oxygen molecule which is not observed on the mono-doped cases. Three different reaction pathways are revealed from the calculations. Due to the large barrier of the O-O breaking, the hydrogenation of the adsorbed oxygen molecule is kinetically more favorable. The free energy change of reaction under different electrode potential is also evaluated. (C) 2017 Elsevier B.V. All rights reserved.