Nitrous oxide emission has been paid more attention in the past few years due to its significant greenhouse effect. Nitrous oxide is a powerful greenhouse gas and its concentration in the atmosphere kept climbing at a constant rate. Nitrous oxide emission from wastewater treatment plant is considered as a major source of anthropogenic input. Conventional nutrient removal processes such as nitrification and denitrification produce nitrous oxide as intermediate or side-product. New treatment technology-Anammox is a promising process for nitrogen removal due to its low energy consumption and high removal efficiency. While being able to significantly decrease carbon dioxide emission, its potential of eliminating nitrous oxide emission has not been studied carefully. In the study, an UASB Anammox reactor was built to investigate nitrous oxide emission source from the reactor. Shock-loading and dissolved oxygen level could affect nitrous oxide concentration in the off-gas. Enrichment of biomass under strict dissolved oxygen control significantly brought nitrous oxide production down from 400 ppm to 5 ppm. With evidence from fluorescence in situ hybridization lab, nitrifier denitrification could be the source of nitrous oxide emission. In the end of study, the average emission of nitrous oxide was only 0.07 % of recovered nitrogen.