Nitrous　oxide　(N2O)　is　a　potent　greenhouse　gas,　which　is　produced　during　nitrifying　and　denitrifying　processes.　Some　factors　and　mechanisms　affecting　N2O　emission　have　been　reported　in　previous　literature,　but　wastewater　biological　nitrification　is　accompanied　by　a　dynamic　process　of　dissolved　oxygen　(DO)　consumption　and　pH　reduction,　it　is　more　meaningful　to　study　the　synergistic　effects　between　DO　and　pH　on　N2O　production.　In　this　study,　the　synergistic　effects　between　DO　and　pH　on　N2O　production　were　investigated　with　real　domestic　wastewater.　The　results　showed　that　high　DO　levels　and　a　high　pH　could　improve　the　oxidation　ratio　of　[GRAPHICS]　and　the　production　ratio　of　[GRAPHICS]　while　effectively　reducing　the　accumulation　ratio　of　N2O.　The　[GRAPHICS]　was　a　prerequisite　for　nitrifier　denitrification;　when　[GRAPHICS]　was　oxidized　completely,　there　would　be　no　N2O　production　and　an　even　higher　concentration　of　[GRAPHICS]　The　pH　factor　is　shown　to　directly　affect　N2O　emission,　although　free　ammonia　and　free　nitrous　acid　which　changed　with　pH　had　no　correlation　with　N2O　emission.　There　were　two　reasons:　(1)　pH　can　influence　the　flow　direction　of　electrons　afforded　by　NH2OH　oxidation;　at　high　pH,　electrons　were　mainly　used　for　combining　H+　and　O-2　(O-2　+　4H(+)　+　4e(-)　=　2H(2)O),　the　accumulation　of　[GRAPHICS]　cannot　be　a　result　of　denitrification,　and　a　higher　DO　can　get　more　electrons　to　prefer　[GRAPHICS]　and　(2)　[GRAPHICS]　was　the　prerequisite　for　NH2OH　oxidation,　since　NH2OH　oxidation　process　was　the　way　to　provide　electrons　for　nitrifier　denitrification.