Urea-SCR (Selective Catalytic Reduction) systems are getting a lot of attention as the most promising NOx reduction technology for heavy-duty diesel engine exhaust. In order to promote an effective development for an optimal urea-SCR after-treatment system, it is important to clarify the decomposition behavior of the injected urea and a detailed reaction chemistry of the reactants on the catalyst surface in exhaust gases. In this paper we discuss experimental and numerical studies for the development of a numerical simulation model for the urea-SCR catalyst converter.As a first step, in order to clarify the behavior of reductants in an urea-SCR converter, two types of diagnostic technique were developed; one is for measuring the amount of NH3, and the other is for measuring the amount of total reductants including unreacted urea and iso-cyanic acid. These techniques were applied to examine the behavior of reductants at the inlet and inside the SCR converter. This revealed i) urea to NH3 conversion rates and spatial distributions of reductants at the inlet of the catalyst, and ii) temporal and spatial profiles of urea to NH3 conversion inside the catalyst.Secondly, we developed a urea-SCR reaction model with detailed surface chemistry for the NH3-NO/NO2 reacting system over a Fezeolite catalyst, which is combined with a simple reaction model for urea-decomposition reactions. The reaction parameters of the urea-decomposition model were determined by the urea to NH3 conversion rates and spatial distributions of reductants, which were directly measured at the inlet and inside the catalyst converter mentioned above. This model successfully predicted gas emissions (such as NOx and NH3) from urea-SCR converters under various vehicle-test conditions.