A sequencing batch reactor operated under anoxic /aerobic condition was used to investigate the activity of hydroxylamine oxidase (HAO) and the production/emission of nitrous oxide during nitrification and denitrification under three different carbon to nitrogen ratios of influent．The results show that when the carbon to nitrogen ratio was 3.5 and 9.5, the HAO activity was averagely 283.77~19.64U/g MLSS and 348.87~17.94U/g, respectively. When the carbon to nitrogen ratio was 6.5, the HAO activity was only averagely 246.45~23.30U/g MLSS. On the whole, the HAO activity in the anoxic period was higher than that in the aerobic period under the three conditions. The trend of variation of the HAO activity was positively correlated with the emission of N₂O, dissolution of N₂O and accumulation of nitrite nitrogen. They were highly consistent in variation in the aerobic period when the carbon nitrogen ratio was 9.5.The results imply: (1) N₂O is mainly produced in the aerobic period during nitrification and aerobic oxidation of hydroxylamine is the major step of N₂O formation; (2) In case of relatively insufficiency of carbon source (eg., C/N=3.5), the release of N₂O depends on the activity of HAO. When the carbon source is relatively sufficient, there was no significant correlation between the N₂O emission and the HAO activity in the anoxic period. This is considered due to the lack of electron acceptors nitrite that hinders the HAO to participate in the reaction. The activity of HAO is relatively higher when N₂O is produced.