The　effect　of　C/N　ratio　on　kinetics　of　nitrite　denitrification,　as　well　as　the　corresponding　N2O　accumulation　and　reduction　were　investigated　in　a　3　L　sequencing　batch　reactor.　The　initial　20　mg　N·L-1　NO2--N　was　obtained　by　adding　NaNO2.　The　C/N　ratios　1.8,　2.5,　3.2　and　4.5　were　reached　by　adding　0,　0.05,　0.1,　0.2　ml　ethanol　respectively.　The　highest　N2O　accumulation　was　333,　200,　245　and　256　μmol·L-1　for　each　C/N　ratio.　The　nitrite　reduction　and　N2O　production　rate　increased　with　the　increase　of　C/N　ratio.　The　reduction　rate　of　N2O　was　about　35-45　mg　N·(g　MLSS)-1·h-1　for　using　internal　carbon　sources　as　electron　donor　and　172　mg　N·(g　MLSS)-1·h-1　for　using　external　carbon　source.　The　accumulation　of　N2O　at　the　initial　of　denitrification　process　was　attributed　to　the　imbalance　between　synthesis　of　NO2-　reductase　and　N2O　reductase.　Under　lower　C/N　ratio,　the　competition　of　nitrite　reductase　and　nitrous　oxide　reductase　for　electron　donor　resulted　in　accumulation　of　N2O.　Under　higher　C/N　ratio,　the　reduction　of　NO2-　and　N2O　happened　simultaneously　and　large　amount　of　N2O　accumulation　may　not　happen.　©　All　Rights　Reserved.