Nitrous oxide (N₂O) is a greenhouse gas that can be released during biological nitrogen removal from wastewater. N₂O emission from a sequencing batch reactor (SBR) for biological nitrogen and phosphorus removal from wastewater was investigated, and the aims were to examine which process, nitrification or denitrification, would contribute more to N₂O emission and to study the effects of heterotrophic activities on N₂O emission during nitrification. The results showed that N₂O emission was mainly attributed to nitrification rather than to denitrification. N₂O emission during denitrification mainly occurred with stored organic carbon as the electron donor. During nitrification, N₂O emission was increased with increasing initial ammonium or nitrite concentrations. The ratio of N₂O emission to the removed ammonium nitrogen (N₂O-N/NH₄-N) was 2.5% in the SBR system with high heterotrophic activities, while this ratio was in the range from 0.14% to 1.06% in batch nitrification experiments with limited heterotrophic activities.