This study dealt with nitrous oxide (N2O) emission and hydroxylamine oxidase (HAO) activity of waste sludge in a nitrification and denitrification process employing three carbon nitrogen (C/N) ratios in a sequencing batch reactor (SBR). The experimental results indicated that N2O emission increased dramatically after the C/N ratio in the sludge increased from 6.5 to 9.3, which was greater than the N2O emission at two other C/N ratios (3.5, 6.5). The HAO activity in the anoxic period was higher with all three C/N ratios than in the aerobic period. The results suggest that N2O was produced primarily in the aerobic period and the main source of the N2O emission resulted from denitrification by nitrifying bacteria and aerobic hydroxylamine oxidation. When a relatively deficient carbon source existed, the N2O emission under anoxic conditions was affected by the HAO activity and vice versa. When the HAO activity was relatively high, it was found that more N2O was released.