Lab　scale　SBR　(Sequencing　Batch　Reactor)　filled　with　carbon　fibre　is　used　to　investigate　the　N2O　production　of　simultaneous　nitrification　and　denitrification　and　the　mechanism　of　the　N2O　production　is　also　discussed.　The　results　show　that　the　efficiency　of　simultaneous　nitrification　and　denitrification　keeps　above　79%.　When　Dissolved　Oxygen　(DO)　is　0.2,　0.4,　1.0,　1.5　mg/L,　the　N2O　production　is　0.005,　0.025,　0.021,　0.025　g/g　N-removed,　respectively,　which　is　lower　than　the　production　of　short-cut　nitrification.　In　one　cycle,　the　N2O　production　increases　with　the　oxidation　of　NH4+-N.　When　the　oxidation　of　NH4+-N　finished,　the　production　of　N2O　decreases　sharply.　During　aeration,　the　correlation　between　the　N2O　production　rate　and　ρ(COD)　concentration　is　found.　After　analysis,　heterotrophic　nitrification　and　aerobic　denitrification　are　suggested　to　be　the　main　source　of　N2O.