Nitrous　oxide　(N2O)　emission　has　been　observed　during　denitrification　of　nitrate　via　nitrite　as　intermediate.　With　a　laboratory-scale　reactor　(2.4　L),　the　N2O　emission　was　characterized　under　a　gradient　of　DO　concentration　from　0　to　0.7　mg/L,　different　ratio　of　nitrite　versus　nitrate　and　different　nitrite　feed　mode.　The　N2O　emission　was　influenced　by　the　level　of　dissolved　oxygen　(DO)　and　nitrite　accumulation.　The　higher　DO　level　and　the　higher　ratio　of　nitrite　versus　nitrate　resulted　in　the　higher　N2O　emission.　Using　nitrite　as　sole　electron　acceptor　at　the　same　loading　rate,　the　sequence　of　N2O　emission　with　three　different　feed　modes　was:　pulse　＞　step-wise　＞　continuous　feed.　The　N2O　emitted　in　pulse　feed　reactors　was　3.1-4.2　and　8.2-11.7　folds　of　that　in　the　step-wise　feed　and　continuous　feed　reactors,　respectively.　With　continuous　feed　mode,　the　impact　of　DO　concentration　on　the　mass　of　N2O　emitted　was　limited　while　the　higher　N2O　emission　occurred　at　the　higher　nitrite　loading　rate.　(C)　2012　Elsevier　B.V.　All　rights　reserved.