To meet the strict emission regulations for diesel engines, an advanced processing device such as a Urea-SCR (selective catalytic reduction) system is used to reduce NOx emissions. The Real Driving Emissions (RDE) test, which is implemented in the European Union, will expand the range of conditions under which the engine has to operate [1], which will lead to the construction of a Urea-SCR system capable of reducing NOx emissions at lower and higher temperature conditions, and at higher space velocity conditions than existing systems. Simulations are useful in improving the performance of the urea-SCR system. However, it is necessary to construct a reliable NOx reduction model to use for system design, which covers the expanded engine operation conditions. In the urea-SCR system, the mechanism of ammonia (NH3) formation from injected aqueous urea solution is not clear. Thus, it is important to clarify this mechanism to improve the NOx reduction model. In particular, the investigation of the hydrolysis of isocyanic acid (HNCO), which is formed as an intermediate product in the process of NH3 formation from urea-water solution, is required. Although previous studies have reported the rate constant of HNCO hydrolysis, these were obtained from indirect rather than direct measurements of HNCO. In this study, the reaction rate of HNCO hydrolysis over Cu-ZSM5 catalyst was measured by generating high-purity HNCO and conducting high-precision HNCO measurements. The reaction rate of HNCO hydrolysis is used in the simulation of the urea-SCR system in order to construct a reliable NOx reduction model for system design.