Formation of N2O during NH3-SCR reaction limits the temperature window of SCR catalyst. In the present study, a series of kinetics experiments were carried out over V-based catalyst to study the N2O formation characteristics at different temperatures. Transient and temperature-programmed reactions show that the thermal decomposition of solid NH4NO3 formed by the reaction of NH3 with NO2 is the main way of N2O formation at low temperatures (below 300 degrees C). At the temperature above 300 degrees C, N-2 selectivity of NO reduction over V-based catalysts decreases with the increase of the gaseous NH3 concentration and the V content of the catalysts. DFT calculations were applied to understand the formation mechanism of N2O at high temperature. The results show that the NH is found to be formed by NH3 oxidative dehydrogenation over V Lewis acid site and subsequently reacts with NO to produce N2O. In this process, the deep oxidation of the adsorbed NH3 is the rate-determined step, which can be aggravated by the high temperature and high V content.