In this study, the effect of tool pin geometry on the mechanical and microstructural changes in the weld region and near it was investigated in the Friction Stir Welding process of butt joint of GOST 4784-97 aluminum alloy, using two types of threaded conical and cylindrical tool pins. To study the microstructure, a metallographic analysis using optical microscope and scanning electron microscope was used, and a tensile and micro hardness tests were used to investigate the effect of pin rotation on the mechanical properties of the joint. The results of the metallographic survey show a smooth and polished surface of the welded specimens with fine threaded cylindrical pin. The EDS analysis indicates the precipitates of aluminum, magnesium, manganese and iron in bright colors and the precipitates of oxygen, magnesium, aluminum, and silicon in the dark colors in the images. The micro-hardness analysis has shown that the highest degree of hardness in the weld region is obtained by coarse threaded cylindrical tool; and its reason was hard mechanical work and high temperature during the welding process, which led to the formation of very fine grains by the dynamic recrystallization mechanism, in the stir region. The tensile strength of all welded specimens was greater than the tensile strength of the base metal; and indicates that there are no imperfections in the welds, as well as an appropriate microstructure in frictional stir welding that improves the mechanical properties of the welded area.