The optimal operating condition of the patented Multi-Stage Corona Discharge System at pulsed mode has been investigated to reduce NOx emission from the exhaust gas of commercial diesel engine vehicles. The design parameters of the system have been tested at numerous operating conditions and upgraded to improve their performance. The compositions of simulated and actual exhaust gas have been measured with a typical exhaust gas analyzer downstream of the system and also analyzed with a FT-IR spectrophotometer. The removal efficiency of NO varied from 20 to 90% depending upon the flow rate and initial NO concentration. The actual residence time of exhaust within the discharge volume played an important role for optimal design. Those results are directly related to the energy consumption of the removal reaction and, ultimately, the design characteristics.