Background:　The　post-denitritation　sequencing　batch　reactor　(SBR)　is　widely-used　and　can　achieve　high　levels　of　nitrogen　removal.　In　this　study　the　effect　of　influent　COD/TN　(total　nitrogen)　ratio　(i.e.　C/N　ratio)　on　nitrogen　removal　performance　was　investigated.　Results:　The　experimental　results　showed　that　polyhydroxybutyrate　(PHB)　was　the　internal　carbon　source　for　denitritation,　so　PHB　degradation　rate　following　first-order　kinetics　was　the　rate-limiting　step　both　for　simultaneous　nitritation-denitritation　(SND)　in　the　substrate　famine　period　of　the　oxic　stage　and　endogenous　denitritation　in　the　anoxic　stage.　Higher　influent　C/N　ratio　resulted　in　more　PHB　fractions　in　microorganisms,　which　facilitated　a　higher　efficiency　of　SND　and　a　faster　endogenous　denitritation　rate　(DNR).　Consequently,　mean　TN　removal　ratio　in　oxic　stage　dropped　from　32.81%　to　8.61%,　and　average　endogenous　DNR　in　the　anoxic　stage　fell　from　1.50　to　0.27　mgN　h-1　gVSS-1,　when　influent　C/N　ratio　changed　from　6.82　to　1.89.　Furthermore,　PHB　fraction　in　the　biomass　did　not　drop　drastically　when　influent　C/N　ratio　dropped　for　a　short-term　period,　which　facilitated　better　resistance　to　shock　loads.　Conclusion:　High　influent　C/N　ration　benefits　nitrogen　removal　in　this　process,　and　an　influent　C/N　ratio　of　4.00　was　suitable　for　advanced　nitrogen　removal.　©　2013　Society　of　Chemical　Industry.