Encouraged by the great promise of metal-nitrogen-carbon (M-N-C) materials in replacing Pt for catalyzing the oxygen reduction reaction (ORR), metal-P species were successfully introduced into carbon matrices in experiments and have exhibited high catalytic activity for the ORR. Here, by means of comprehensive density functional theory (DFT) computations, we investigated the origin and the mechanism of the ORR occurring on Fe- and Co-P-embedded graphenes. Our computations have revealed that the Fe- and Co-P4 moiety-embedded graphenes possess good stability and high chemical reactivity for O2 activation, thus facilitating the subsequent ORR steps, and a more efficient 4e pathway in both acidic and alkaline media is more energetically favorable. Furthermore, by analyzing the computed free energy profiles, the Fe-P4 species-embedded graphene is a more efficient electrocatalyst for the ORR in an alkaline medium than the Co-P4 species-embedded graphene. Our DFT computations will be useful for gaining deeper insight into the high activity of metal-P species.