In　this　study,　nitrous　oxide　(N2O)　production　during　biological　nutrient　removal　(BNR)　from　municipal　wastewater　was　reported　to　be　remarkably　reduced　by　controlling　copper　ion　(Cu2+)　concentration.　Firstly,　it　was　observed　that　the　addition　of　Cu2+　(10-100　mu　g/L)　reduced　N2O　generation　by　54.5-73.2　%　and　improved　total　nitrogen　removal　when　synthetic　wastewater　was　treated　in　an　anaerobic-aerobic　(with　low　dissolved　oxygen)　BNR　process.　Then,　the　roles　of　Cu2+　were　investigated.　The　activities　of　nitrite　and　nitrous　oxide　reductases　were　increased　by　Cu2+　addition,　which　accelerated　the　bio-reductions　of　both　nitrite　to　nitric　oxide　(NO　(2)　(-)　-＞　aEuro　parts　per　thousand　NO)　and　nitrous　oxide　to　nitrogen　gas　(N2O　-＞　aEuro　parts　per　thousand　N-2).　The　quantitative　real-time　polymerase　chain　reaction　assay　indicated　that　Cu2+　addition　increased　the　number　of　N2O　reducing　denitrifiers.　Further　investigation　showed　that　more　polyhydoxyalkanoates　were　utilized　in　the　Cu2+-added　system　for　denitrification.　Finally,　the　feasibility　of　reducing　N2O　generation　by　controlling　Cu2+　was　examined　in　two　other　BNR　processes　treating　real　municipal　wastewater.　As　the　Cu2+　in　municipal　wastewater　is　usually　below　10　mu　g/L,　according　to　this　study,　the　supplement　of　influent　Cu2+　to　a　concentration　of　10-100　mu　g/L　is　beneficial　to　reduce　N2O　emission　and　improve　nitrogen　removal　when　sludge　concentration　in　the　BNR　system　is　around　3,200　mg/L.