Nitrous oxide (N2O) is emitted from wastewater treatment processes. It is known as a greenhouse gas that contributes to global warming (over 200 times more per molecule than carbon dioxide) and to the destruction of the ozone layer. It is therefore of great importance to develop technology that can suppress N2O emission. The effects of an anoxic period on N2O emission and nitrogen removal were investigated in an actual domestic wastewater treatment plant. When operated with intermittent aeration, most of the N2O was emitted into the atmosphere during the aerobic period. N2O emission from the intermittent process was estimated to be 0.43-1.89 g N2O person(-1) year(-1). Maintaining a dissolved oxygen (DO) concentration of over 0.5 mg l(-1) during the aerobic period resulted in the complete conversion of the influent NH4-N to NO3-N and a 60-min anoxic period was sufficient for denitrification to be completed. The findings show that an optimum combination of aerobic and anoxic conditions and their suitable control are very important for improving nitrogen removal efficiency and controlling N2O emission.