Today, the use of Interstitial-free steels (IF) in the automotive industry is expanding. One of the main challenges of these steels is their connection and final welding quality. Therefore, in this study, the friction stir spot welding of galvanized interstitial free steel was studied on sheets with thickness of 0.8 mm. Also, the effect of different welding parameters including the tools rotation speed (710, 900, 1120 and 1400 rpm) and the dwells time (3, 6 and 9S) on the mechanical properties of the joints were investigated. With doing shear tensile test and study result, tool rotation speed 1120 rpm and the dwells time 9S was selected as the optimal parameter. Titanium dioxide (TiO 2 ) nanoparticles with an average diameter of 30 nm and different weight percent (0.002, 0.005 and 0.009 g) were used to improve the mechanical properties of the weld and inhibit the grain growth in the stir zone in the friction stir spot welding was used. Optical microscopy, stereography, scanning electron microscopy, energy emission spectrometry and X-ray diffraction were used to study the macroscopic and microstructural properties of different weld regions. To evaluate the mechanical properties of the weld point, done a shear tensile test and micro hardness test. Analysis of surfaces fracture and evaluation of fracture behavior of samples were also studied using stereography and scanning electron microscopy. In order to study the corrosion behavior of the weld surfaces, a potentiodynamic polarization test and immersion test done in the 0.1M sodium chloride solution and sulfuric acid. The results of this study showed that increasing the tool rotation speed from 710 rpm to 1120 rpm reduced grain size in the stir zone to less than 2.4 ?m. However, the increase in the tool rotating speed from 1120 rpm to 1,400 rpm has an adverse effect and has led to the grain growth in the stir zone. Also, the addition of a sufficient amount of titanium dioxide nanoparticles in the stir zone controlled the grain growth, adding a value of 0.005 g of these particles reduced the grain size in the stir zone to less than 0.5 ?m. The data obtained from the hardness test indicated an increase of hardness of about 30 Hv as a result of the presence of titanium dioxide nanoparticles. In addition, the presence of titanium dioxide nano-powders led to an increase in failure load of about 780 N. However, the addition of more than 0.005 g of reinforcing particles results in the agglomerate and disintegration of these particles in some regions of the stir zone, which reduces the failure load of the joints. In examining the fracture mode of the samples obtained from the shear tensile test, three fracture mode were found, including interfacial mode, combined fracture mode and fracture in the TMAZ region. The fracture mode in the TMAZ region, has a higher energy absorption capacity and is more favorable for industrial applications. The results of the potentiodynamic polarization and immersion tests indicated a higher resistance to corrosion of the base metal than to different welding points. Also, the density of the corrosion current (i corr ) was less than that of the various welding samples. In addition, the results of potentiodynamic polarization tests showed that the sample containing 0.005 g of titanium dioxide nanoparticles, had higher corrosion resistance and less corrosion density than other welding samples. Keywords: friction stir spot welding, galvanized interstitial free steel, microstructure, mechanical properties, fracture mode, potentiodynamic polarization tests.