The　diversity　of　denitrifying　bacteria　in　the　aerobic　tank　of　a　simultaneous　nitrification　and　denitrification　system　was　studied　by　building　nirS　cloning　library　chain　reaction　(PCR)　and　molecular　cloning.　75　clones　were　randomly　selected　from　the　cloning　library　to　be　sequenced　and　blasted.　74　clones　belonged　to　three　different　bacteria　groups,　including　β-proteobacteria,　γ-proteobacteria　and　uncultured　bacterium.　β-proteobacteria　was　the　dominant　bacterial　group　in　the　aerobic　tank,　which　accounted　for　54.41%　in　the　library;　γ-proteobacteria　was　the　second　group　and　accounted　for　25%.　12　OTU　were　selected　from　the　74　sequenced　clones　for　phylogenetic　analysis　using　MEGA　software,　the　Thauera　was　the　main　denitrifier　genera　in　the　system.　©,　2015,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.