NO oxidation using O-3 is studied, including the product composition under different conditions and the oxidation selectivity in complex flue gas. The O-3/NO molar ratio determines the type of oxidation products. N2O5 is generated when the O-3/NO molar ratio is greater than 1, and the N2O5 yield increases with the improvement of the O-3/NO molar ratio. High temperature leads to the decomposition of NO3 and the decrease of the N2O5 yield. When NO3 decomposition is caused in the high-temperature region, the increase of the residence time is not beneficial to the generation of N2O5. In addition, water vapor in the flue gas is beneficial to the formation of N2O5 at low temperatures (60-90 degrees C), but SO2, CO, and HCl in the flue gas show little influence on the NO oxidation selectivity.