At　normal　temperature　of　19　degrees　C　+/-　1　degree　C　and　seed　sludge　from　the　system　of　conventional　nitrogen　removal　via　nitrate　composed　of　both　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB),　the　competition　and　optimization　of　AOB　and　NOB　was　investigated　in　a　lab-scale　SBR　treating　domestic　wastewater.　The　reactor　was　operated　under　the　aeration　rate　of　40　L/h　with　the　average　DO　concentration　below　1.0　mg/L,　and　combined　with　the　control　of　aeration　time,　i.e.,　aeration　was　stopped　before　the　pH　＂ammonia　valley＂　was　detected.　After　AOB　became　pre-dominant　nitrifying　bacteria　(nitrite　accumulation　rate　NO2(-)-N/NOx(-)-N　reached　50%),　the　aerobic　durations　were　gradually　prolonged　from　3　h　to　4　h　and　5　h　to　improve　the　ammonia　removal　efficiencies　and　enhance　AOB　dominant.　By　the　above　operational　pattern,　nitrogen　removal　via　nitrite　was　successfully　achieved　with　NO2(-)-N/NOx(-)-N　over　95%.　Fluorescence　in-situ　hybridization　(FISH)　analysis　showed　that　AOB　was　accounted　for　9.97%　of　total　biomass.　The　on-line　control　of　aerobic　duration　was　an　effective　method　to　maintain　the　partial　nitrification　and　the　NH4(+)-N　removal　efficiency　was　above　97%.　The　research　results　also　proved　that　at　normal　temperatures,　for　the　seed　sludge　from　the　conventional　nitrogen　removal　via　nitrate,　AOB　would　not　become　dominant　nitrifying　bacteria　only　by　the　aeration　time　control　without　the　DO　restriction.