Display omitted •NH2OH has a strong inhibition on NOB.•Inhibition of NOB by 5 mg-N/L NH2OH is reversible.•NH2OH addition caused production of NO and N2O to increase.•Inhibition of Nitrospira by NH2OH could be related to NO production. This study investigated a strategy for hydroxylamine (NH2OH) addition for promoting the conversion of complete nitrification to partial nitrification in a sequencing batch reactor (SBR). The results showed that continuous dosing of 5 mg-N/L NH2OH into a complete nitrification reactor for 16 days led to an increase in the nitrite accumulation ratio (NAR) from 0.22% to 95.08% and a significant enhancement in the accumulation of NO and N2O in the liquid. The maximum concentration of NO in each cycle rose with the increase of NAR during NH2OH addition. With the stopping of NH2OH addition, the partial nitrification disappeared progressively in 21 days. The analysis for microbial community showed that Nitrospira was the main NOB and its relative abundance decreased with NH2OH addition and recovered after the cessation of NH2OH addition. Accordingly, NH2OH has a significant and reversible inhibition on Nitrospira and its essence might be related to NO toxicity.