In the past years, non-precious transition metal and nitrogen co-doped graphene electrocatalysts for oxygen reduction reaction (ORR) have attracted great attention because of their high catalytic activity and stability. In this work, the reaction mechanism for ORR on FeN3 doped divacancy graphene (FeN3-Gra) has been investigated by using the density functional theory. The results showed that FeN3-Gra is thermodynamically stable. The ORR elementary reactions could take place within a small region around FeN3 and its adjacent eleven carbon atoms. HOOH is unstable on the catalyst surface, indicating that ORR is a four-electron process. O-2 hydrogenation is much easier than its dissociation. The most favorable pathway is the OOH hydrogenation to form O + H2O. The formation of the second water molecule is the rate-determining step with an energy barrier of 0.48 eV. This barrier is much lower than 0.80 eV for pure Pt, also lower than 0.56 eV for FeN4 doped divacancy graphene. In addition, the calculated Tafel slope is also smaller than pure Pt in both low and high overpotential regions. The above results indicated that FeN3-Gra is a potential ORR catalyst. (C) 2018 The Electrochemical Society.