In this study, the effect of plate positioning on mechanical properties of dissimilar lap joints was investigated by friction stir spot welding (FSSW) process. The determination of the welding parameters plays an important role for the weld strength. For the effective use of the dissimilar aluminum joints, the FSSW must have an adequate strength. The quality of the joint was evaluated by examining the characteristics of the joining efficiency as a result of the lap-shear tensile test. Four process parameters were selected: the tool rotation speed, dwell time, tool plunge depth, and tilt angle.