A new process to oxidize NO and SO2 in flue gas simultaneously was presented, on the basis of atmospheric chemistry research. The mechanism of this process was analyzed. Methods to initiate the chain reactions were investigated, and the thermal decomposition of additives in flue gas at medium temperature was suggested to be promising, because of its low cost and high efficiency. With nitric acid as the additive, numerical simulations and experiments were developed, to investigate the possibility of applying this new process in flue gas. Numerical and experimental results showed good coherence. Both results showed that the simultaneous oxidation of NO and SO2 can be realized under the proper conditions. There was a temperature window for the oxidation of NO and SO2, and 750 K was determined to be an optimum temperature. The oxidation ratio of SO2 increased fast initially and then slowly as the HNO3 concentration increased, whereas the oxidation ratio of NO decreased slowly as the HNO3 concentration increased. The oxidation ratio of NO decreased as the NO concentration increased. The oxidation ratio of SO2 increased initially and then decreased as the NO concentration increased, which,showed a competition between the chain reactions and the termination reactions. Surface reactions are one consumption source of key radicals, and minimizing the wall surface effect by coating inert material enhanced the simultaneous oxidation of NO and SO2.