Abstract: 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.