Due to its relatively high-performance and compactness, the pulsed-discharge de-NOx process is expected to be an advanced technology to suppress air pollution. Adequate guidelines for optimum operation of the pulsed-discharge de-NOx process have not yet been established however. In this study, we numerically analyze the process subjected to several hundred high-voltage pulses and investigate the effects of by-products and ammonia injection on the de-NOx performance. The electron collision process to produce OH and N radicals to remove NOx is analyzed by the Boltzmann equation for the energy distribution of discharge electrons. The chemical reaction process between the unstable radicals and NOx including combustion flue gas is calculated by considering a total of 1004 rate equations for electron collision and chemical reaction processes and a total of 101 chemical species. In a case without ammonia injection, both the NxOy removal efficiency and the de-NOx energy consumption rate to remove NxOy change with an increase in repeated pulse number because electrons produced by the discharge attach to accumulated by-products, such as H3O+(H2O)2, followed by a decrease in radical concentration, i.e., a decrease in oxidative and reductive removal reaction rates. In the case with ammonia injection, the removal efficiency increases and the electric energy consumption rate decreases with an increase in ammonia concentration because removal reactions suc- h as NO→NO2→HNO3→NH4NO3 and NO→N2 are promoted. When excess ammonia is injected, the de-NOx performance declines because the NH2 radical produced by electron collision with ammonia reacts with NO2 and forms relatively stable N2O. In a case where HNO2 is considered NxOy, the de-NOx performance is also assessed. © 2004 American Institute of Physics.