The　performance　of　partial　nitrification　via　nitrite　and　microbial　community　structure　were　investigated　and　compared　in　an　SBR　by　operating　under　different　DO　conditions.　The　reactor　achieved　stable　partial　nitrification　with　nitrite　accumulation　ratio　of　above　90%　both　at　high　and　low　DO　concentrations　by　using　the　real-time　aeration　duration　control.　Compared　with　high　DO　(ρ(DO)=3±0.5　mg/L),　distinct　simultaneous　nitrification　and　denitrification　(SND)　via　nitrite　was　carried　out　at　low　DO　(ρ(DO)=0.5±0.1　mg/L)　despite　with　lower　ammonia　oxidation　rate.　The　average　efficiency　of　SND　at　high　DO　and　low　DO　concentrations　was　9.5%　and　45.4%,　respectively.　The　maximum　SND　efficiency　reached　86%　under　low　DO　concentration.　Fluorescence　in　situ　hybridization　(FISH)　analysis　showed　that　AOB　(ammonia　oxidizing　bacteria)　became　the　dominant　nitrifying　bacteria　and　NOB　(nitrite　oxidizing　bacteria)　was　gradually　washed　out　from　the　reactor　when　stable　nitrite　accumulation　was　established.　The　sizes　of　AOB　and　NOB　were　8~10%　and　lower　than　0.5%　at　the　end　of　high　DO　operation　period,　respectively.　After　the　operation　under　low　DO,　AOB　gradually　increased　to　10~12%　but　NOB　decreased　into　negligible　level.　It　can　be　concluded　that　the　real-time　aeration　duration　control　has　positive　effect　on　the　sludge　population　optimization　under　both　high　and　low　DO　conditions.　Keeping　DO　at　a　low　level　is　not　only　beneficial　to　the　wash-out　of　NOB　and　the　increasing　of　nitrite　accumulation　ratio,　but　also　favorable　to　the　enhancement　of　SND　efficiency.