NO in exhaust gas is the major source of air pollutants, causing photochemical smog and acid rain. The corona discharge plasma process for removal of NO has been studied intensively for increasing its removal efficiency. Therefore, the development of diagnostic tools measuring major species and minor radicals produced in the plasma is important to characterize the removal process. OH is one of the oxidizing radicals that can initiate the NO removal process. The end product (HNO3) of the NO oxidation process is produced by the OH and NO2 reaction. Therefore the OH concentration is closely related to the removal efficiency of NO and NO2. In the study we measure the oxidizing radical OH emission spectrum in a nonthermal plasma reactor and investigated H2O, H2O2 and C2H4 additive effects on NO/NO2 reduction and OH emission intensity. For the measurement of the low intensity of OH emission in a corona discharge, we made a cylinder-wire type plasma reactor that concentrated the corona emission in the center of the reactor.