Sheet metal clinching process is a forming-based method for joining sheet metal parts. To ensure sufficient joint strength, it is necessary to design the forming tool optimally. This paper deals with numerical and experimental study of the clinching process of steel sheets of dissimilar thicknesses using a fixed die in order to optimize the geometric parameters of the clinching tool. In this study, using the orthogonal experimental design (OED) method and finite element analysis in Abaqus software, the important input parameters of tool design including punch radius R_P, die cavity depth P_m, die groove width L_m and punch face angle P_BA were optimized in order to achieve the highest clinch strength F as the target variable. The upper and lower sheets used in this study are 1.5 and 2 mm in thickness, respectively, and made of DX51D galvanized steel, manufactured according to EN10346/00 by Mobarakeh Steel Company. After running the experiments designed based on the OED in the computer, the optimal values of R_P= 2.6mm, P_m= 1.4mm, L_m= 1.2mm and P_BA= 1° and F= 2319N were obtained. Next, a clinched joint tool was designed and fabricated based on the optimum geometric parameters. The evaluation and comparison of clinch geometry and tensile strength obtained from optimum design simulation and the experimental counterparts demonstrated very close correlations.