The nitrogen oxides (NOx) emission standard has become more stringent in the past decade due to the critical global air pollution. In order to reduce the amount of NOx generated from automobiles, improving the performance of selective catalytic reduction (SCR) systems which can reduce NOx emissions becomes an important topic in the automotive industry. Due to the large gas flow rate in commercial vehicles, the packaging constraints and the sizes of SCR catalysts in the market, the SCR systems installed in the commercial vehicles consist of a number of SCR catalysts, either in parallel or in series, and connected by pipes and chambers. There are three major factors which can improve the performance of a SCR system - creating even gas flow rate, uniform speed through the catalysts, and lower total pressure loss. The first two can help operate the SCR catalyst efficiently and even life cycle, at the same time the lower total pressure loss can improve the performance of the engine system. Besides the studies to improve the performance of the single SCR catalyst such as adjusting the thickness of the coating layer or changing the material for the coating layer, we focus on the performance improvement for the entire SCR system by modifying the system arrangement or layout. In this study, we will give quite a few strategies and examples for the layout modifications and examine the impacts from these modifications. By these modifications, the total pressure loss can reduce up to 29% and the flow uniformity index can improve by about 10%. Numerical simulation is adopted in this study and the flow solver is ANSYS Fluent Ver.14.