In this paper, the stability and the oxygen reduction reaction (ORR) catalytic activity of single transition metal atom doped g-C3N4 catalysts, M-C3N4 (M = Mn, Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, Au), were investigated in detail by performing density functional theory (DFT) calculations. The results of binding energy reveal all M-C3N4 are thermodynamically stable. Further dynamic calculations demonstrate they are also dynamically stable except Au-C3N4. Then, through comparing the value of overpotentials, we found that most of M-C3N4 exhibit no ORR catalytic activity except for Ag-C3N4 and Pd-C3N4, both of which have somewhat catalytic properties but still inferior to Pt(111). It may be caused by the strong adsorption between ORR intermediates (OOH, O, OH) and M-C3N4. We further preformed DFT calculation for the high-valent metal complexes of g-C3N4 (M-OH-C3N4) and the significant enhancement of activity is obtained. Due to the additional OH group, the overall adsorption energies of ORR intermediates on M-OH-C3N4 have been decreased and become more close to those on Pt(111), and ORR mechanisms have also been changed. In addition, the overpotentials of ORR on Ni-OH-C3N4 and Cu-OH-C3N4 are much close to that on the Pt(111), indicating that they possess the catalytic activity comparable to precious Pt catalyst. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.