Laboratory scale models simulating a mixed liquor circulating (MLC) process and a sequencing batch reactor (SBR) process were operated with a high ammonium volumetric loading to study the possible nitrous oxide (N2O) emission from them. The N2O emission from the MLC process was found to depend on the DO level in the aerobic reactor. When the DO level was high, the emission was negligibly small. The SBR reactor emitted N2O in a quantity as high as 40 percent of the removed nitrogen. Most of the emission occurred during the low DO period in the aeration stage.