To clarify the reaction mechanism of selective non-catalytic NO reduction using urea solution, we investigated the reaction kinetics. The mechanism of urea decomposition, which is the first step in the reaction, was considered by comparison of calculated and experimental results. In addition, the main reaction route was determined by sensitivity analysis. The model in which I mol each of ammonia and isocyanic acid are formed from urea explained the experimental results better than the model in which 2 mol of ammonia are formed. Sensitivity analysis indicated that the OH radical converts ammonia into NH2, Which reacts with NO to form NNH and finally N-2. We also clarified the mechanism of decomposition of ammonia and isocyanic acid after the reductionof NO concentration.