A DeNO(x) process using a DC corona discharge was investigated experimentally. A mixture system of N-2/O-2/NO was used as a test gas. The compositions such as NO, NO2, N2O and so on were analyzed with Fourier transform infrared spectroscopy and an NOx meter. It was found that the characteristics of NO removal by corona discharge differed remarkably whether or not oxygen exists in the mixture. In regard to the spectrum of light emission from the corona discharge in N-2 atmosphere or N-2/O-2 mixture, some N-2 bands were detected. N-2 dissociation into atomic N and N-2 radical in the corona discharge field was conjectured. Furthermore, ozone was yielded by the corona discharge in the case of the N-2/O-2 mixture. Ozone gas from an ozonizer was added into the N-2/O-2 mixture without corona discharge to investigate the effect of O-3 on the characteristics of NOx removal by corona discharge. In the case of the N-2/NO mixture, the process of NO reduction was mainly controlled by N-2 radicals excited by the corona discharge. On the other hand, in the case of the N-2/O-2/NO mixture, NO was oxidized by ozone generated from the corona discharge and converted to NO2 and N2O5.