The　start-up　and　stability　of　partial　nitrification　in　a　continuous　flow　bioreactor　was　investigated　in　this　study.　Nitrogen　loading　rate　(NLR)　was　set　to　be　the　central　examined　factor　and　partial　nitrification　performance　of　AO　system　was　monitored　with　its　variation.　Results　from　experiments　indicated　that　NLR　was　the　most　direct　factor　leading　to　nitrite　accumulation,　and　inhibition　of　free　ammonia　(FA)　and　free　nitrite　acid　(FNA)　had　significant　effect　on　partial　nitrification.　The　gap　between　the　specific　growth　rate　of　ammonia-oxidizing　bacteria　(AOB)　and　that　of　nitrite-oxidizing　bacteria　(NOB)　leads　to　the　AOB　enrich　and　NOB　eliminate　in　the　system　fed　by　high　ammonia　concentration　inflow.