The aim of this study is to improve weld characteristics employing a new proposed method in adding SiC reinforcement nanoparticles to friction stir spot welds (FSSW). 6061-T6 aluminum alloy plates 3 mm in thickness and SiC nanoparticles 45 to 65 nm in diameter were used in FSSW. Design of experiments, S/N ratio analysis and optimization of joint ultimate strength were conducted using Taguchi method. In this way, the amount of nanopowder, tool rotational speed and dwell time were considered as input parameters to optimize joint ultimate strength. A confirmation test was also conducted which validated optimal measure predicted by Taguchi. The fracture mode of each examined specimen was specified by an investigation of their force-displacement graphs and failed nugget surfaces. The specimen microstructures were examined next and grain size and nanoparticles distribution in weld region were evaluated. Achieved results justified the effect of design parameters on ultimate strength and microhardness of the joint. Energy dispersive x-ray spectroscopy (EDS) indicated that nanoparticles assembled in grain boundaries during dynamic recrystallization and prevented the growth of grain size. Incorporation of SiC nanoparticles in FSSW increased joint ultimate strength and average hardness in stir zone (SZ) up to 28% and 24%, respectively.