In the present work, an interstitial free (IF) steel was subjected to friction stir spot welding (FSSW). The silicon carbide (SiC) nanopowders with the average diameter of 25 nm were also added to the joints in order to enhance the mechanical properties and also to prevent the grain growth in the stir zone (SZ). The effects of tool rotating speed (800, 1250, 1600 and 2000 rpm) and holding time (4, 6 and 8 s) on the welds quality were studied. In addition, the influence of sheet thickness (0.7 and 1 mm) on the welds quality was investigated. Optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies along with the results of the microhardness measurements, tensile shear analyses and peel test revealed that adding the SiC nanopowders to the welds led to the significant decrease in the grain size of the SZ as well as the remarkable increase in the mechanical properties and hardness of the produced welds. Keywords: Electron microscopy, Mechanical characterization, Steel, Welding, Failure, Grain growth