Abstract: In SBR process,rapid startup of partial nitrification was not only associated with the number of cycles needed for startup,but also with nitrification time(nitrification rate) of each cycle.When temperature was 21²3℃,the effect of dissolved oxygen(DO) on partial nitrification indicated that DO mass concentration of 0.25¹.25 mg/L was beneficial to startup of partial nitrification in which ρ(DO)of 0.25⁰.75 mg/L was superior for rapid startup.The effects of ρ(DO) of 0.25⁰.50 and 0.50⁰.75 mg/L on rapid startup of partial nitrification were fair,mainly because when ρ(DO) was 0.25⁰.50 mg/L,although competitive advantage of AOB was more significant,doubling time of AOB increased.During the operation of partial nitrification in which ρ(DO) was about 1.50¹.75 mg/L,long-term steady operation achieved by reducing ρ(DO) to 0.50⁰.75 mg/L at intervals.Molecular biology was used to analyze the sludge sample taken from steady operation period.Resultsof general primers analysis showed that the proportion of AOB and NOB in total bacteria was 22.50%and 3.75%respectively,especially,Nitrosomonas sp.was the dominant bacterial genus in AOB,and its proportion of the total bacteria was 17.50%.