Laser-induced fluorescence (LIF) technique aided by intensified CCD light signal detection and fast digital image processing is demonstrated to be a useful diagnostic method for insitu observation of the discharge-induced plasma-chemistry processes responsible for NO_X (NO + NO₂) decomposition occurring in non-thermal plasma reactors. In this paper a method and results of the LIF measurement of two-dimensional distribution of the ground-state NO molecule density inside a DC positive streamer corona reactor during NO removal from a flue gas simulator [air/NO(up to 300 ppm)] are presented. The obtained results showed that the corona discharge-induced removal of NO molecules occurred not only in the vicinity of the plasma region formed by the corona streamers and in the downstream region of the reactor but also in the upstream region of the reactor, i.e. before the flue gas simulator has entered the plasma region. This information, obtained owing to the LIF technique, is important for the understanding of the plasma-chemistry processes responsible for NO_X decomposition in non-thermal plasma reactors and for optimising their performance.