Abstract—Although technology development for electric vehicles and hydrogen vehicles is important from mid to long-term perspective. Efforts to improve the emission and efficiency of internal combustion engine (currently leading the automobile market) vehicles, are important for improving air quality and greenhouse gas. It is expected that the development of various technologies to reduce nitrogen oxides (NOx) will also be accelerated in line with the global resolution of environmental problems and the strengthening of environmental regulations. Among them, Selective Catalytic Reduction (SCR) is a technology that converts NOx into harmless N2 and H2O by using ammonia as a reducing agent, one of the air pollutants. Due to various advantages, it is the most representative technology for reducing NOx among the developed technologies. Recently, SCR technology is attracting attention due to stricter NOx emission regulations around the world. In this paper, the design of the mixer and mixer pipe of the exhaust gas post-treatment device SCR was optimized through flow analysis, and the effect on the flow and back pressure of exhaust gas was studied by conducting an experimental study using a diesel engine and an engine dynamometer. According to the design change of the pipe and mixer, the SCR Uniformity Index was confirmed to be 96.1% and 97.4% before and after the improvement, respectively, confirming that the flow of combustion gas is uniformly distributed. As the exhaust flow rate increased, the back pressure increased rapidly.