The　partial　nitrification　(PN)-Anammox　process　was　considered　as　one　of　the　most　efficient　and　economical　method　to　remove　ammonia　from　wastewater　without　organic　carbon　consumption.　In　this　process,　PN　is　the　key　step　since　it　should　provide　stable　effluent　with　suitable　[GRAPHICS]　ratio　for　the　subsequent　Anammox.　In　this　study,　PN　was　successfully　started-up　during　30　d　at　ambient　temperature　(25　degrees　C)　in　a　continuous　flow　membrane　bioreactor　(MBR),　by　gradually　increasing　ammonia-loading　rate　(ALR)　under　oxygen-limited　condition.　Finally,　the　system　achieved　an　ammonia　oxidation　rate　of　0.52　kg　m(-3　)d(-1)　and　an　excellent　effluent　with　appropriate　[GRAPHICS]　ratio　as　1.1-1.3.　ALR　was　indicated　as　the　indispensable　controlling　factor,　and　the　applied　alkalinity　with　the　combination　of　high　ALR　inhibition　and　oxygen　limitation　provided　the　selective　washout　of　nitrite-oxidizing　bacteria　(NOB)　and　an　active　aerobic　ammonia-oxidizing　(AerAOB)　population.　Techniques　of　fluorescence　in　situ　hybridization　and　clone　sequencing　were　utilized　simultaneously　to　study　the　microbial　characteristics　in　the　stable　PN　system.　The　results　demonstrated　that　small　amounts　of　NOB　were　present　and　not　active,　while　AerAOB　predominated　in　the　MBR-PN　system,　and　Nitrosomonas-related　AerAOB　were　considered　to　be　responsible　for　the　satisfactory　PN　performance.