N2O　formation　was　studied　using　Sequencing　Biofilm　Batch　Reactor　(SBBR)　by　applying　different　DO　(Dissolved　Oxygen)　gradient　in　the　system.　Simultaneous　nitrification　and　denitrification　(SND)　process　was　achieved　at　different　DO　concentrations.　Total　N2O　productions　were　0.495,　0.423,　0.843　and　2.01　mg-N/L　respectively　in　accordance　with　DO　concentrations,　which　were　controlled　at　1.0,　2.0,　2.5　and　3.0mg/L.　Nitrite　accumulation　increased　with　the　increasing　of　DO　concentration.　The　activity　of　N2O　reductase　was　inhibited　by　higher　nitrite,　which　resulted　in　N2O　emission.　Besides,　nitrifier　denitrification　process　during　which　nitrite　as　electron　acceptor　and　ammonium　as　electron　donor　was　responsible　for　part　of　N2O　emission.　When　DO　was　controlled　at　2.0　mg/L,　more　than　97%　of　ammonium　was　removed　by　SND　process　and　N2O　emission　was　lower　than　any　other　processes.　It　showed　that　both　lower　and　higher　DO　concentrations　could　suppress　SND　process　and　a　balanced　SND　may　reduce　N2O　production.　©　2011　IEEE.