NOx, a generic term for the nitrogen oxides generated from combustion in the presence of nitrogen, is a serious threat to human health. This study examined the removal of NOx using ammonia (NH3) and ozone produced using a silent discharge method. The effects of temperature and residence time on NOx removal with NH3 injection in a double reactor system were investigated. An increase in temperature resulted in higher levels of O3 decomposition, whereas the maximum particle formation in the form of ammonium nitrate (NH4NO3) was achieved when both reactors were kept at 180 oC. NH3 and O3 injection in large quantities and NO in smaller amounts with a residence time of 10.2 s resulted in the maximum particulate formation. In contrast, when an excess of NH3 was supplied, it resulted in N2O formation due to the formation of NH2 radicals generated from a reaction of NO2 with NH3. In addition, 100% NO removal was achieved regardless of the residence time. Kinetic simulations indicated the possibility of moisture being the limiting reactant.