Li–O2 batteries have been a subject of extensive studies in the past few decades. However, the oxygen reduction reaction (ORR) mechanism is still unclear on air cathodes and needs to be concretely explored. In this work, by means of density functional theory computations, we systematically investigated the ORR and initial Li2O2 nucleation processes on the surface of pristine and N-doped graphene in Li–O2 batteries. The in-plane pyridinic N-doped graphene is more effective in facilitating the nucleation of Li2O2 clusters than pristine or graphitic N-doped graphene. The overpotential of the rate-controlling step for Li2O2 nucleation decreases with the growth of Li2O2 clusters, and the cluster growth after (Li2O2)2 will follow the process Li → LiO2 → Li2O2 on all considered substrates. Our results should promote the understanding of ORR processes on N-doped graphene catalysts and shed more light on the design and optimization of air cathodes for Li–O2 batteries.