The　sludge　from　enrichment　of　Nitrobacter　and　Nitrospira　was　used　as　a　research　object　and　batch　tests　were　performed.　The　inhibitory　effects　of　hydroxylamine　on　Nitrobacter　and　Nitrospira　under　the　same　pH　and　different　hydroxylamine　concentration　gradients,　the　same　hydroxylamine　concentration,　and　different　pH　gradients　were　investigated.　The　results　showed　that　under　the　same　pH　condition,　the　activity　of　Nitrobacter　decreased　with　increasing　hydroxylamine　concentration.　Under　the　same　hydroxylamine　concentration　(HA=5　mg•L-1)　at　a　higher　pH　environment　(pH　≥　7.5),　hydroxylamine　produced　more　free　hydroxylamine　(FHA)　and　the　inhibitory　effect　on　Nitrobacter　was　improved.　At　a　low　pH　environment　(pH≤7),　ionic　hydroxylamine　promoted　the　activity　of　Nitrobacter.　The　inhibitory　effect　of　hydroxylamine　on　Nitrospira　was　limited.　When　pH=7.5　and　hydroxylamine　concentration　was　45　mg•L-1,　the　relative　activity　of　Nitrospira　was　82%.　The　NOB　growth　rate　kinetics　model　and　the　non-substrate　inhibition　linear　equation　were　used　to　describe　the　effect　of　FHA　on　Nitrobacter　and　Nitrospira　activity.　The　coefficient　of　determination　R2　was　0.90　and　0.94,　respectively.　FHA　may　be　the　main　reason　for　inhibiting　the　activity　of　Nitrobacter　and　Nitrospira.　©　2020,　Science　Press.　All　right　reserved.