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.