In partial nitritation process, the sludge concentration was higher than 4000 mg x L(-1), DO was less than 0.2 mg x L(-1), the temperature of 15-29 degrees C, the reactor HRT of 4.6 h, and the circulation ratio was 75%, not only the microbial biomass reached the highest value but also the ratio of nitrite to ammonium of the effluent could be controlled at approximate 1.0 by adjusting the dissolved oxygen concentration. A quantitative PCR and the 16S rRNA genes clone library results demonstrated that ammonia-oxidizing bacteria were the dominant among nitrite-oxidizing bacteria at low dissolve oxygen concentration, that the bacteria promoted the running of the partial nitritation. Compared with conventional nitrification systems, in partial nitritation process, Nitrobacter and Nitrospira could not be detected among the system, and abundance of Planctomycetes was higher in partial nitritation process than that of conventional nitrification systems. Phylogenetic analysis of bacterial amoA gene sequences suggested that ammonia-oxidizing bacteria of the partial nitritation system was dominated by Nitrosomonas sequences, so partial nitritation is to be feasible technically and economically.