Diesel engine is being used widely in many industrial fields, as it provides merits in the aspects of higher thermal efficiency and less CO₂ emission. However, NOx regulations for diesel engines are being strengthened and it is impossible to meet the emission standard without aftertreatment systems such as SCR (Selective catalytic reduction), LNC (Lean NOx catalyst), and LNT (Lean NOx trap). Among the NOx reduction aftertreatments, Urea-SCR system is known as the most stable and efficient method to solve the problem of NOx emission. But this device has some issues associated with the ammonia slip phenomenon which is occurred by shortage of evaporation and thermolysis time, and that makes it difficult to achieve uniform distribution of the injected urea. Therefore, this study has focused on the mixing enhancement between urea and exhaust gases to enhance the efficiency of the SCR equipped in catalytic muffler by changing injector angle to improve the spray uniformity of the urea water solution. Finally, it can be found that various parameters such as the injector and injection angles and an exhaust gas temperature significantly affect the urea evaporation and mixing with exhaust gases, and therefore, optimization of these parameters are required.