The effect of dissolved oxygen(DO)and ratio of N2 and O2 on N2O yield during biodenitritation process using nitrate solution as electron acceptor and ethanol as carbon source was investigated in a sequencing batch reactor by controlling gas rate at 0.3 L·min-1 and at low oxygen concentration.The results show that DO could decrease significantly activity of denitritation bacteria.The aeration rate falls off from 18.12 mg N·（g MLSS）-1·h-1to 11.37 mg N·（g MLSS）-1·h-1

while N2O yield increases from 0.23 mg·L-1 to 1.74 mg·L-1.The phenomena can be attributed to:(1)higher concentration of NO-3N leads to decrease of denitritation rate as well as accumulation and emission of N2O;(2)both synthesis and reducing N2O activity of enzymes cause inhibition of DO. A key factor for decreasing N2O emission during biodenitritation process is to lessen the content of DO in bioreactor.