The　formation　and　emission　of　N2O　in　various　nitrogen　removal　processes　for　treatment　of　domestic　sewage　were　investigated　using　oxic-anoxic　SBR　systems　and　a　continuum　aerobic　submerged　biofilm　reactor　system　SBBR,　from　which　complete,　shortcut　and　simultaneous　nitrification　and　denitrification(SND)　processes　were　realized,　respectively.　Meanwhile,　the　effect　of　DO　on　the　SND　efficiency　and　N2O　yield　in　SBBR　was　also　studied.　The　results　showed　that　for　both　complete　and　shortcut　N　removal　processes　the　formation　of　N2O　was　in　nitrification　process,　while　denitrification　one　was　favorable　for　decrease　of　N2O　emission.　N2O　yield　was　4.67　mg　·　L-1,　6.48　mg　·　L-1　and　0.35　mg　·　L-1　for　complete,　shortcut　and　simultaneous　nitrification-denitrification　processes,　respectively.　The　main　reason　for　the　formation　of　N2O　was　the　accumulation　of　NO2--N　during　N　removal　process.　A　part　of　AOB　could　be　denitrified　between　NH4+-N　as　electron　donor　and　NO2--N　as　electron　acceptor　under　low　DO　level,　which　led　to　the　formation　of　N2O　finally.　The　formation　of　N2O　in　simultaneous　nitrification　and　denitrification　process　at　various　DO　conditions　showed　that　in　SBBR　system,　the　minimun　N2O　yield　could　be　achieved　by　the　control　of　DO　concentration　and　the　efficiency　balance　of　nitrogen　removal　in　both　nitrification　and　denitrification　processes.