In order to reduce oxides of nitrogen (NOx) emanated from a diesel engine, a comprehensive urea selective catalyst reduction (SCR) DeNOx catalyst was modeled in which numerical simulations were used as a complementary tool for the experimental investigations to make the design decisions, and hence shorten the development process. In this approach, relevant conversion reactions were studied in 1D model, and the parameters obtained in this way were transferred to 3D simulations. According to the results of the study, the conversion of NO and NO2 increased with the increase in monolith solid temperature. With the increase in the ratio of NO2/NO, the conversion of NO, NO2 and NOx increased resulting in maximum reduction of NOx at the ratio of 1; beyond this ratio, the conversion of NO2 and NOx decreased; however, NO continued to be converted till the ratio was 1.8. The conversion of NOx decreased with the increase in space velocity.