In this paper, a transient thermal analysis model for Diesel fuel systems is presented. The purpose of this work is to determine the fuel temperature at various locations along the system, especially inside the tank and at the returned fuel inlet to the tank. Due to the fact that the fuel level is continuously changing during any driving condition, the fuel mass inside the tank is also continuously changing. Consequently, the fuel temperature will change even under steady driving or idle conditions, therefore, this problem should be analyzed using transient thermal analysis models. Effective thermal management requires controlling the surface temperature of the fuel tank, fuel lines and the fuel temperature at the fuel return line as well as inside the tank [1, 2]. Based on the thermal analysis results, it is possible to determine the major source of heat input at several locations of the fuel system. If necessary, a cooling module can be designed to bring down the returned fuel temperature to an acceptable limit. Other thermal protection options that are investigated, in this work, include shielding the fuel tank, fuel supply and return lines and modifying the fuel system materials and dimensions. The results of the mathematical models are compared to test data from physical vehicle testing with respect to various driving scenarios A Design For Six Sigma (DFSS) process is applied in this work for evaluation robust design and determination of most influential design factors.