In this study, the partial nitrification process was started-up successfully in a membrane bioreactor (MBR). The influence of temperature and DO was investigated by sequencing operation of membrane bioreactor. The preferred values were proved as 35 degrees C and 0.3-0.5mg/L, respectively, and were indicated as indispensable controlling factors. In order to increase the sludge concentration, new seed sludge was added into the reactor, which caused the absolute destruction of the reactor performance. The results of reactor experiments showed that the free ammonia (FA) concentration of 74 mg NH(3)/L, as the influent ammonium concentration of 600 mg N/L, was a useful and effective factor to recover the partial nitrification performance. Fluorescence in situ hybridization analysis indicated that nitrifiers hybridizing with NIT3 and NSR1156 were present and active in MBR, which were then eliminated under high FA concentration. The microbiological community analysis further provided the necessary biological information for the realization of partial nitrification.