Diesel engine has a good fuel economy and high durability and used widely for power source such as heavy duty in the world. On the other hand, it is required to reduce NOx (Nitrogen Oxides) and PM (Particulate Matter) emissions further from diesel exhaust gases to preserve atmosphere. The urea-SCR (Selective Catalytic Reduction) system is the most promising measures to reduce NOx emissions. DPF (Diesel Particulate Filter) system is commercialized for PM reduction. However, in case that a vehicle has a slow speed as an urban area driving, a diesel exhaust temperature is too low to activate SCR catalyst for NOx reduction in diesel emissions. Moreover, the diesel exhaust temperature becomes lower as a future engine has less fuel consumption.The purpose of this study is reduction of NOx emission from a heavy-duty diesel engine using the Urea SCR system at the low temperature. The performance of NOx reduction especially at low temperature was investigated to the influence of NOx/NO ratio at the inlet of Urea-SCR catalyst and the effect of NH₃ adsorption on the Urea-SCR catalyst. The SCR catalysts were coated by the zeolite with different active metals. The performance of NOx reduction at low temperature was observed by the effect of NOx/NO ratio at the inlet gas of Urea-SCR catalyst and the NH₃ adsorption on the Urea-SCR catalysts.The NOx reduction efficiency was tested using simulated gas experiment. In this study, the improvement of NOx reduction catalyst at the low temperature is discussed about tested new material catalysts. These catalysts were selected from the viewpoints of NH₃ and NO adsorption amounts. Drastic NOx reduction efficiency improvement at low temperature (around 200 degree C) would be achieved using copper zeolite catalyst. The next step, emissions reduction effect was discussed from the results of the engine experiments under steady-state and transient conditions. The latest results and problems were reported with the results from the simulated gas experiments and engine experiments.