Abstract: The bacterial diversity in the ASBR and SBR of the combined process, i.e. combined ozonation-ASBR/SBR sludge reduction with simultaneous nitrogen and phosphorus removal process was studied by molecular biological PCR-DGGE method before and after sludge reduction. The results indicated that the dominant bacterial communities in the ASBR were Firmicutes bacterium, β-proteobacterium, γ-proteobacterium and Bacteroidetes bacterium. The bacterial richness in SBR after sludge reduction was slightly increased compared with that before sludge reduction, but with high similarity (85%). The dominant bacteria diversity of SBR was not influenced significantly by the ozone dosage for sludge reduction. There were Firmicutes bacterium, Candidate division TM7, ammonia-oxidizing bacterium, Bacteroidetes bacterium, β-proteobacterium, Denitrifying bacterium, Pseudomonas sp and Nitrosococcus halophilus Nc4 in SBR both before and after sludge reduction. However, the number of Bacteroidetes bacterium, β-proteobacterium, Denitrifying bacterium and Nitrosococcus halophilus Nc4 in SBR after sludge reduction was slightly higher than that before sludge reduction.