Friction stir spot welding (FSSW) is derived from friction stir welding process, which is an unconventional joining process. This process exhibits great capability to continue substitute of distinct tip joining process similar to riveting and impeding post welding process. The joining of dissimilar AA2524 -T3 and AA7051-T7451 aluminum plates of 3 mm thickness was carried out using friction stir spot welding (FSSW) technique. Current study considers the tool geometry; rotational speed and joining force as main performance parameters. Three types of tools with various designs have been utilized to analyze the process. An impact of process parameters on mechanical strength and micro structural analysis of lap joints were investigated. The most significant parameter such as tool shoulder diameter to pin diameter ratio shows the greatest influences the joint shear resistance followed by tool rotational speed. The results show that the temperature obtained during processing performs a weighty role in finding the microstructure and features of the processed plates. Failure modes are observed under different load conditions. The welded surfaces were analyzed by scanning electron micrographs. © 2017 Elsevier Ltd. All rights reserved.