This　study　focused　on　the　elucidation　of　the　mechanisms　of　nitrogen　and　phosphorus　removal　in　simultaneous　nitrification-denitrification　and　phosphorus　removal　(SNDPR)　systems　enriched　with　phosphorus　accumulating　organisms　(PAOs).　An　extended　anaerobic　(aeration　duraction:　180　min)/low　aerobic　(dissolved　oxygen:　0.5~1.0　mg/L)　sequencing　batch　reactor　(SBR)　fed　with　domestic　wastewater　was　studied　for　SND　(simultaneous　nitrification-denitrification)　efficiency　of　SNDPR　under　different　C/N　ratios　(3,　4,　8,　and　11)　adjusted　by　adding　sodium　acetate　into　the　domestic　waster.　The　experimental　results　showed　that　the　phosphorus　removal　efficiency　was　not　affected　by　influent　C/N　ratios　with　an　effluent　PO43--P　concentration　lower　than　0.3　mg/L,　which　might　be　caused　by　the　enriched　PAOs　capable　of　achieving　a　simultaneous　aerobic　phosphorus　uptake　and　denitrifying　phosphorus　uptake　at　the　low　aerobic　phase.　In　contrast,　effluent　NH4+-N　concentration　increased　with　the　C/N　ratio　and　the　effluent　NO3--N　concentration　increased　with　the　decrease　of　C/N　ratio.　The　consumption　of　chemical　oxygen　demand　(COD)　for　exogenous　denitrification　increased　with　the　decease　in　C/N　ratio.　The　storage　capacity　of　PHAs　(poly-hydroxyalkanoates)　for　SND　was　reduced,　but　its　utilization　of　PHV　(Volatile　fatty　acids)　was　increased.　SND　efficiency　reach　the　peak　value　of　50.8%　at　the　C/N　ratio　of　4~8,　and　total　nitrogen　(TN)　removal　efficiency　reached　80.8%　at　the　C/N　ratio　of　8.　©,　2015,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.