The single atom supported by carbon nitride (g-C3N4) materials, as efficient substitutes for platinum catalysts, recently have attracted extensive attention. Owing to poor electron mobility, g-C3N4 suffers from unavoidable energy loss, while g-C3N4 with crystalline carbons (g-C3N4/C) can enhance electron transfer. In this study, single Cu atom doped g-C3N4 (Cu/g-C3N4) and g-C3N4/C (Cu/gC(3)N(4)/C) are investigated by using the density functional method. Our study indicated that they are stable thermodynamically. ORR intermediates have weakened adsorption energies in Cu/g-C3N4/C compared with Cu/g-C3N4. ForCu/g-C3N4, the rate-determining step is O-2 hydrogenation to form OOH with an energy barrier of 0.40 eV, similar to 0.39 eV for Pd/g-C3N4. Compared with Cu/gC(3)N(4), Cu/g-C3N4/C shows higher catalytic activity. For Cu/g-C3N4/C, the energy barrier at the rate-determining step is 0.15 eV, smaller than 0.40 eV for Cu/g-C3N4, and much smaller than 0.80 eV for Pt (111). Meanwhile, the predicted working potential is 0.57 V for Cu/g-C3N4/C, also larger than 0.22 V for Cu/g-C3N4. Thus, the introduced crystalline carbons on Cu/C3N4/C enhances not only electron conduction but also catalytic activity. (C) 2019 The Electrochemical Society.