In order to realize efficient nitrogen removal and N2O emission reduction, air lift circulation bioreactors were applied to study the relationship between activated sludge aggregation state and N2O emission characters on the basis of high nitrogen removal performance. The nitrification/denitrification activity of different microbial aggregates was evaluated by key enzyme action ratio method. Combined with correlative theoretical analysis, the optimal aggregation state with efficient nitrogen removal and N2O emission reduction was selected. According to the results, different activated sludge aggregation state in SND would lead to significant difference of N2O emission amount (> or = 40%). The smaller aggregates (< or = 0.9 mm) with moderate compactness could keep high SND efficiency (> or = 70%) and achieved N2O emission reduction. In experimental defined system, the optimal diameter of aggregates was in the range of 0.45-0.9 mm, which could obtain higher nitrification activity [ammonia-oxidizing bacteria activity was 0.17 mg x (g x min) (-1), nitrite-oxidizing bacteria activity was 0.74 mg x (g x min) (-1)] and denitrification activity [NO3(-) -N consumption rate was 0.47 mg x (g x min) (-1), NO2(-) -N consumption rate was 0.22 mg x (g x min) (-1)]. Compared to the control group, N2O accumulated emission amount in the reactor with aggregates of 0.45-0.9 mm could be realized reduction more than 32.55%.