The effects of Hg2+ on microbial community structures of aerobic activated sludge were investigated by nested PCR-DGGE techniques. The results showed that the microbial community of each group might have obvious change compared with the group without Hg2+ in the inflow, especially when the Hg2+ concentration was 46.87mg/L in the inflow, and the microbial community changed obviously with the extend of operation time. The variance of microbial species become obvious in each activated sludge group with the extend of operation time. Compared with the group without Hg2+ in the inflow, the dominant microbial species of each activated sludge group changed, and the dominant microbial species of the group whose concentration of Hg2+ increased gradually in the inflow had the biggest changes after 60 days. Hg2+ in the inflow may stimulate the growth of Actinomycetes after an operation period of 30 days.