To disclose the mechanism of N-removal in a full-scale landfill leachate treatment system (Shenzhen), community structure of total bacteria and nitrifier in this system were explored by PCR-DGGE and real-time PCR. The influence of environmental factors on microbial community structure was analyzed. The results show that 91. 4% of TOC and 99. 9% of NH⁺_4-N were removed during the whole process. The quantitative assay show that the numbers of total bacteria,AOB, and NOB had no significant relationship with the concentrations of TOC and ammonia (p > 0. 05). Dissolved oxygen (DO) had no effect on the number of AOB, but had great influence on the number of NOB. Microbial community structure and DMS analysis show that the DO concentration affected the community structure of total bacteria and NOB, while ammonia concentration determined the community structure of AOB.