Abstract: Lab scale SBR (Sequencing Batch Reactor) filled with carbon fibre is used to investigate the N₂O production of simultaneous nitrification and denitrification and the mechanism of the N₂O production is also discussed.The results show that the efficiency of simultaneous nitrification and denitrification keeps above 79%.When Dissolved Oxygen (DO) is 0.2、0.4、1.0、1.5mg/L,the N₂O production is 0.005、0.025、0.021、0.025g/g N-removed,respectively,which is lower than the production of short-cut nitrification.In one cycle,the N₂O production increases with the oxidation of NH₄⁺-N.When the oxidation of NH₄⁺-N finished,the production of N₂O decreases sharply.During aeration,the correlation between the N₂O production rate and ρ(COD) concentration is found.After analysis,heterotrophic nitrification and aerobic denitrification are suggested to be the main source of N₂O.