In　order　to　optimize　the　population　structure　of　nitrifying　bacteria,　several　factors　were　analyzed　in　this　paper　to　make　partial　nitrification　start　up.　Lab-scale　SBR　was　employed　to　treat　the　domestic　wastewater　of　low　COD/TN　ratio.　Steady　partial　nitrification　can　be　achieved　through　real-time　control　in　32　cycles　with　dissolved　oxygen　(DO)　1.8　+/-　0.1　mg/L　and　SRT　7　days　at　30　degrees　C.　The　nitrite　accumulation　rate　(NO2+-　N　/NOx+-　N)　was　over　90%　under　the　premise　of　TN　removal　ratio　above　95%.　In　the　next　64　days,　the　system　was　also　successfully　operated　at　ambient　temperature　(20　similar　to　24　degrees　C).　Fluorescent　in-situ　hybridization　(　FISH)　analysis　showed　that　the　population　of　ammonia　oxidizing　bacteria　(AOB)　increased　by　58.9%　while　nitrite　oxidizing　bacteria(NOB)　decreased　by　83.2%　during　32　cycles.　NOB　can　be　gradually　washed　out　by　the　process　control　of　DO　and　aerobic　duration　based　on　DO　and　pH　on-line　sensors　so　steady　partial　nitrification　and　nitrogen　removal　efficiency　can　be　achieved　and　enhanced,　respectively.