The reduction of NOxfrom diesel engine exhaust gas by superimposing barrier discharge plasma reactors is experimentally investigated. The experiments are conducted for the applied voltages from 0 to 24 kV, electrode gap distance from l.5 to 5.0 mm, and the flow rates from 0.2 to 2 L/min at initial NOxconcentration 1,700 ppm. The results show that NOxreduction increases with increasing discharge power and electrode gap distance. The superimposing barrier discharges under out-of-phase operation mode shows best result, where maximum NOxreduction rate is approximately 60% at operating electric power 10 W.