In the present investigation, 2 mm sheets of AA1050-O and AA6061-T4 were joined using friction stir spot welding (FSSW). The influences of tool rotational speed, dwell time and plugging depth on the tensile-shear force of FSSW lap joints were determined. The FSSW were performed using a tool with a taper pin profile and concave shoulder surface. The analysis of variance (ANOVA) approach was used as a statistical design of experiment technique to set the optimal FSSW parameters that maximize the tensile-shear load of AA1050-O/AA6061-T4 dissimilar lap joints and also to determine the contribution of each FSSW parameter. The results revealed that tool rotational speed has the most significant statistical and physical effect on the tensile-shear load of AA1050-O/AA6061-T4 dissimilar lap welded joints. The optimum FSSW process parameter that maximize the tensile-shear load are at tool rotational speed, dwell time and plunging depth of 710 rpm, 7 sec. and 3.05 mm, respectively.