The effects of semi-spherical pin on mechanical properties and microstructure of 7 mm thick friction stir welded dissimilar aluminum alloys AA7075 and AA5083 were investigated. Threee different pin profiles were used to weld three joints of dissimilar AA7075 and AA5083 Al alloys. Semi-spherical pin, cylindrical pin, and pin with round groove. The friction stir welding was performed at 700 rpm rotational speed and 25 mm/min welding speed. Defect-free FSW joint was obtained by using a simple tool with semi-spherical pin. Meanwhile, the crack-like defect is formed in the joints produced by the tools with cylindrical pin, and pin with round groove. The usage of FSW tool with semi-spherical pin (large frictional area of pin) produces a friction stir weld joint with higher tensile strength. When the tool with semi-spherical pin is applied, the grains size was smaller than that when using the other tools.