The　influences　of　HRT,　C/N　ratio　on　simultaneous　nitrification　and　denitrification　(SND)　and　the　rule　of　pH　in　a　sequencing　batch　reactor　(SBR)　were　investigated　while　treating　low　C/N　synthetic　municipal　wastewater.　The　results　showed　that　the　function　of　SND　became　more　outstanding　and　the　removal　rate　of　TN,　NH4+-N　was　improved　greatly　by　lengthening　HRT　appropriately;　when　the　C/N　increased,　the　denitrification　effect　of　the　simultaneous　nitrification　and　denitrification　would　be　improved;　.At　the　same　time,　the　variation　of　pH　value　was　well　related　to　ammonia　oxidation.　So　that　judgment　on　the　ending　of　nitrification　and　denitrification　could　be　based　on　the　inflection　point　on　the　varied　curve　of　pH,　and　thus　reducing　aeration　and　mixing　time　for　the　purpose　of　energy　saving.　©　(2011)　Trans　Tech　Publications,　Switzerland.