Recently, there has been increasing interest in catalysts with smaller volume for a Urea Selective Catalytic Reduction (SCR) system especially for use in heavy duty vehicles. In this study, several new concepts were developed in order to improve the deNOx performance of the SCR catalysts over a wide range of operating temperatures and this resulted in a compact SCR system.First, the urea decomposition process in diesel exhaust gas was elucidated. Several kinds of urea decomposition catalysts were investigated and the material which showed the best performance in NH3 (ammonia) formation was used to improve the low temperature performance of Cu-zeolite catalysts.Second, the method of reducing the amount of NH3 slip was investigated. It is well known that the amount of ammonia slip after the Urea-SCR system must be under 10 ppm and therefore materials with lower NH3 slip are preferred. The smaller the amount of NH3 slip, the larger the amount of urea that can be injected into the system and this leads to higher NOx conversion.A material that could store NH3 at temperatures even higher than 450 degrees C was added to Cu-zeolite in order to enhance the adsorption of NH3 at high temperatures and this consequently reduced the amount of NH3 slip.The concepts described above were combined and this resulted in the development of a high deNOx performance Cu-zeolite catalyst that also made it possible to reduce the volume of the SCR catalyst as well.