Tests were performed to investigate nitrifying bacterial communities and activities in aerobic biofilm reactors with different temperature conditions, denaturing gradient gel electrophoresis (DGGE) based on polymerase chain reaction targeting 16S rRNA and amoA gene, fluorescence in situ hybridization (FISH) and dehydrogenase activity (DHA).T1, T2 and T3 reactors operated at different temperatures (5, 10 and 30 ℃, respectively) were set up in the thermostat and acclimated. Nitrification was considerably limited in T1 and T2 reactors. DGGE revealed specific genera of ammonium-oxidizing bacteria (AOB) and some Nitrosomonas genera endured at the low temperatures. FISH revealed a decreased distribution ratio between AOB and nitrate-oxidizing bacteria at 5 ℃, and showed that the decrease of AOB also affected the nitrification failure in the aerobic biofilm reactor. The mean attached biomass of the T1, T2 and T3 reactors was 69.6, 80.6 and 112.9 mg/L, respectively, and the 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride dehydrogenase activity of the respective reactors was 73.6, 87.4 and 134.2mgO2/g VSS/day. The results demonstrate that a low temperature condition in an aerobic biofilm reactor decreases the attached biomass, distribution ratio and activity of nitrifying bacteria, and produces a change in the composition of the AOB species, which results in the failure of nitrification.