Recently, Urea-SCR system is used for reducing NOx emission from diesel engine. In the Urea-SCR system, aqueous urea is injected into the exhaust gas flow where it evaporates and decomposes to NH₃. It is necessary to clarify the generation process of NH₃ in high temperature atmosphere for the improvement of NOx reduction performance. In this study, the generation behavior of NH₃ and the mass reduction of solid urea and 32.5wt% urea solution were measured in a high temperature vessel. As a result, the start of NH₃ generation became early and the maximum NH₃ concentration increased with increasing temperature in the decompositions of both samples. The start of NH₃ generation in the urea solution decomposition was later than that in solid urea decomposition owing to the evaporation of water. Moreover, the amount of mass reduction rate was increased when N₂ was passed through the vessel. It seemed that the reaction of urea was accelerated because reaction products were removed by the N₂ flow. As for the residue after decomposition, the mass of residue decreased with increasing temperature in the decompositions of both samples. The residues of them were decreased when N₂ was passed through the vessel under lower atmospheric temperature conditions.