The paper obtains the procedure of design and analysis of the geometric parameters of the aerodynamic spoiler on the cabin of a semi-trailer truck. The procedure was included in order to obtain the optimal geometric shape and position of the spoiler, which would reduce the drag force of a semi-trailer truck. The procedure of optimizing the geometric shape and position is based on Design of Experiments and Response Surface Methodology. The research includes physical experiments in a wind tunnel and Computational Fluid Dynamics simulation. A scaled model of a semi-trailer truck is used as object of the optimization process.