Abstract: Activated sludge in pre-anoxic and anaerobic reactors in the starting-up of Carrousel 2000 oxidation ditch was sampled from April to August in 2005, and their genomic DNA was extracted directly and purified. Then the 16S rDNA genes (V₃ region) were amplified and gradient gel electrophoresis denatured (DGGE). Microbial diversity was studied through comparing the DGGE fingerprints. The results showed that the maximum coefficient similarity is 68.2% in April, 57.7% in May to June and 83.6% in August in anoxic reactor, and it is from 64.8% to 62.7% and then to 71.5% in anaerobic reactor at the similar time. The microbial community in the two reactors shows the trend of increasing at first and then decreasing. The similarity between the two reactors becomes decreasing, the maximum coefficient similarity in the three phrases is 80.5% (starting-up phase), 60.8% (middle adjustment phase), and 59.3% (later adjustment phase). In conclusion, the domestication of microbial community in aerobic activated sludge in Carrousel 2000 oxidation ditch was about four months.