The　aim　of　this　paper　is　to　investigate　the　performance　and　optimization　of　an　biological　nitrogen　removal　process　enhanced　by　the　anoxic/oxic　four　stages　step-feeding　process　under　sufficient/insufficient　influent　carbon　source.　The　concept　of　influent　flow　distribution　ratio　(lambda)　was　introduced　according　to　the　performance　of　the　step-feed　process　for　inflow　splitting　and　volume　distribution.　Under　the　condition　of　sufficient　carbon,　the　uniform　influent　flow　distribution　ratio　was　proposed　from　the　standpoint　of　which　is　easy　for　process　control,　operation　optimization　and　management.　Under　the　condition　of　different　influent　C/N　ratios,　the　optimal　influent　flow　distribution　ratios　(lambda(opt))　were　determined,　by　trial-and-error　method.　The　results　showed　that　more　than　95.81%　of　the　total　nitrogen　removal　efficiency　could　be　achieved　by　step-feeding　process　with　a　fixed　influent　C/N　ratio.　A　model　was　introduced　to　derive　the　relationships　of　total　nitrogen　removal　efficiency　with　lambda　and　influent　C/N　ratio.　When　the　influent　carbon　source　was　insufficient,　however,　the　lambda(opt)　between　two　adjacent　stages　should　not　be　uniform.　The　results　also　showed　that　the　four-stage　step-feeding　process　with　lambda(opt)　of　2.0:2.1:2.5:3.4　could　significantly　improve　the　total　nitrogen　removal　efficiency.　The　results　obtained　in　this　study　will　help　to　optimize　the　design　and　the　operation　of　BNR　systems.　(C)　2008　Elsevier　B.V.　All　rights　reserved.