In this work, first-principle methods are employed to build thermodynamic models for both the pure and sulfur atom modified g-C3N4 photocatalysts. Three possible mechanisms of oxygen evolution reaction (OER) following four one-electron pathway are investigated. The hydroxyl (OH) species as a key intermediate is found to strongly interact with the catalyst and its newly observed stability indeed significantly affects the overpotential of OER. On pure g-C3N4, the first removal of proton from water, the rate-determining step, can not become surmountable at room temperature until an overpotential of 0.88V (2.11V vs SHE) is appended, in accord with the experimental observation that water photooxidaton hardly occurs on g-C3N4 without any modification. Interestingly, the sulfur doping not only leads to a different reaction mechanism but also lowers the overpotential, consistent with the experimental finding that the reaction rate for OER could be further enhanced by sulfur-modified g-C3N4. Our theoretical results provide useful insights for designing better anodes to achieve high OER activity on graphitic carbon nitride based photocatalysts. (C) 2015 Elsevier B.V. All rights reserved.