Butt weld friction stir welding of steel sheets with thicknesses of less than 1 mm has been known as one of the most important challenges in recent years. In this work, butt weld friction stir welding of IF (Interstitial free) steel with the thickness of 0.7 mm is studied experimentally. For this purpose the effects of rotational and travel speeds of the tool on the strength and hardness of welded joints is investigated. Mechanical strength of the joints is evaluated with uniaxial tensile test. Also, in this paper the microstructure evolutions of welded joints of IF steel sheets with the thickness of 0.7 mm are studied. The results show that with increasing in rotational speed and decreasing in travel speed of the tool, the tensile strength of welded joints will be increased. In addition, with increasing in rotational and travel speeds of the tool, the hardness of welded zone is increased. The results of microstructural investigations show that the microstructure of welded zone is improved with increasing in rotational and travel speeds of the tool. The results of EBSD analysis show that no significant changes have been occurred in the left side of stir zone in comparison to center of stir zone. Also, it is shown that the fraction of high angle grain boundaries in the center of stir zone has been increased than the left side of stir zone (from 25 % up to 48 %).