Partial　nitrification　(PN)　process　was　successfully　developed　in　a　plug　flow　reactor　fed　with　low　strength　sewage　at　ambient　temperature　(18-21.5　℃).　For　better　detecting　the　efficiency　and　mechanism　of　the　partial　nitrification　process　in　the　PN　reactor,　ammonia,　nitrite,　and　nitrate　were　detected　firstly,　subsequentially　the　functional　bacterial　community　in　the　reactor　was　also　investigated　at　microbial　level.　The　sludge　microstructures　were　detected　by　scanning　electron　microscopy　(SEM),　and　the　microbial　characteristics　was　studied　via　fluorescence　in　situ　hybridization　(FISH),　denaturing　gradient　gel　electrophoresis　(DGGE),　cloning　and　sequencing　analyzing.　The　DO　concentration　was　maintained　between　0.1　to　0.6　mg/L,　to　enhance　AOB　competition　superiority.　After　80　days　continuous　operation,　PN　reactor　achieved　nitrite　accumulation　rate　of　100%　and　effluent　NO2--N　to　NH4+-N　ratio　achieved　1.11.　SEM　results　showed　that　spherical　bacteria　were　the　predominant　bacteria　in　partial　nitrifying　sludge.　FISH　results　showed　the　proportion　of　AOB　and　NOB　were　37.3%　and　4.4%,　respectively.　Bacterial　DGGE　and　sequencing　results　indicated　that　six　dominant　bacteria　coexisting　in　the　PN　reactor,　among　which　Nitrosomonas　sp.　were　the　main　AOB　species.　Those　multiple　functional　bacteria　contributed　to　the　nitrogen　removal　in　PN　reactor.　©　2016,　Harbin　Institute　of　Technology.　All　right　reserved.