Abstract　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　0.88　V　(2.11　V　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.　©　2015　Elsevier　B.V.