In order to establish a design rule of the plasma reactors, a fundamental investigation was carried out. The results show that important points of the design criteria were flow speed of gas and power input to the discharge tubes. An engineering model of plasma reactor for 100 kVA diesel engine generating system was designed, constructed and tested. The reactor is composed of 3 blocks, 30 discharge tubes with 1 m long and they are driven by a high voltage inverter power source. The test results show that the fundamental data were applicable to the engineering model reactor and the required energy yield of NO oxidation process was almost attained. A scale up rule has also been confirmed