Sustainable nitrogen removal from wastewater is of increasing significance in the future, aiming to achieve savings of energy consumption and operational cost as well as minimized emission of greenhouse gases. Under such a context, the proposal of N2OPNA is developed to give first insights into nitrous oxide (N2O) production, a potent greenhouse gas, from the mainstream partial nitritation and Anammox (PNA) process. The influencing factors such as sludge particle size, dissolved oxygen, nitrite, hydroxylamine and inorganic carbon will be investigated using enriched granular ammonia oxidizing bacteria (AOB) and Anammox bacteria. Isotopic technique will be applied to shed some light on the transformation of N2O production pathways under varying operational conditions. Based on the process and isotopic data obtained from experiments, an integrated mathematical model for predicting N2O production in PNA system with the consideration of electron transfer, energy balance and bacterial growth will be established. The findings in this proposal would be beneficial for the design and operation of full-scale wastewater treatment plant with the aim of N2O mitigation.