The NOx reduction performance of three types of nonthermal plasma (NTP) reactors is compared. An experiment using simulated gas is first conducted. Subsequently, those reactors are applied to a NOx aftertreatment system for a stationary diesel engine. Twelve surface discharge elements are placed in a different arrangement in each of those reactors. The results show a possibility that NOx reduction performance depends on the gas flow surrounding the elements. The reactor with the best arrangement shows a NOx reduction energy efficiency of 19.4 g (NO₂)/kWh. The application of one of those reactors to the NOx aftertreatment system indicates that NOx reduction performance is not deteriorated by CO₂. The aftertreatment system shows more than 50% NOx removal efficiency for at least 8 h with a system energy efficiency of 15.4 g (NO₂)/kWh.