Abstract: Biological aerated filter (BAF) has been widely used in treatment of wastewater and reclaimed water. To date, little research has been done to control N ₂O production in BAF. N ₂O production from municipal wastewater at different air-water ratio of 9.8:1,5.0:1,2.5:1 respectively was investigated in biological aerated filter (BAF). The obtained results show that N $\begin{array}{c}{H}_4^{+}\end{array}$ -N and TN removal efficiency decreases with the decrease of air-water ratio. Under the condition of air-water ratio of 9.8:1,5.0:1,2.5:1, the percent of N ₂O emission from BAF was 418.0306,247.9406,123.0638×10 ^-6, respectively, indicating that N ₂O production in BAF can not be ignored. N ₂O-N dissolved in the mixture at different air-water ratio increased first and then decreased along the filter layer, which was consistent with the variation of N $\begin{array}{c}{O}_2^{-}\end{array}$ -N concentration. Therefore, N ₂O might be produced during nitrification. Considering nitrification efficiency, TN removal efficiency and N ₂O emission control, the optimal air-water ratio is 5.0:1.