Sequencing　batch　reactors　(SBR)　are　used　to　investigate　the　greenhouse　N2O　production　in　complete　and　short-cut　denitrifications　under　different　ρ(COD)/ρ(N).　Using　ethanol　as　carbon　source　to　control　ρ(COD)/ρ(N),　which　are　1.56,　2.83,　4.56,　6.01,　and　10.0　in　complete　denitrification　and　1.51,　2.45,　3.33,　4.13,　and　9.7　in　short-cut　denitrification.　Results　show　that　6.01　and　4.13　are　the　optimum　ρ(COD)/ρ(N)　in　complete　and　short-cut　denitrifications　respectively　with　little　N2O　production.　One　gram　of　mixed　liquor　suspended　solids　(MLSS)　can　reduce　0.0779　nitrate　or　0.0899　nitrite　per　day.　The　specific　denitrification　rates　increase　with　increasing　ρ(COD)/ρ(N).　However,　there　is　no　remarkable　impact　with　adding　organic　matter　when　the　carbon　source　is　abundant.　When　the　carbon　source　is　insufficient,　N2O　production　in　short-cut　denitrification　is　obviously　larger　than　that　in　complete　denitrification,　and　the　amount　topped　0.607　mg/L　at　ρ(COD)/ρ(N)　of　3.33.　High　N2O　production　under　low　ρ(COD)/ρ(N)　operations　is　mainly　due　to　the　inhibition　of　nitrite　to　the　activity　of　nitrous　oxide　reductase.