The Light metals such as aluminum and magnesium alloys find extensive use in land and air transport vehicles, electronics, computer and sporting goods industries. In order to reduce weight and thus save fuel, many studies are carried out on the development of new engineering materials, which have lightweight and high fatigue strength, especially for the automotive and aviation industry. One of the first metals remembered in this field is magnesium, which has lower density. In contrary to these advantages, joining of light metal alloys with fusion based welding methods have some problems. Since the fusion welding of light metal sheets is difficult, friction stir welding (FSW) method frequently used for joining of these. Studies has shown that shoulder profile of welding tool, which apply pressure on material, affects the welding quality in FSW method. In order to understand these effects various shoulder profile has been studied and understood. FSW method has been investigated and used for joining of materials since about 50 years. In this paper, the pinless shoulder profile designs of stir tool and the mechanical properties and fatigue strength of magnesium and aluminum alloy sheets, which are joined with these stir tools, are aimed to investigate. In order to realize this, shoulder profiles were designed and manufactured accordingly. Subsequently, magnesium and aluminum alloy sheets for automotive and aviation applications were joined with friction stir spot welding (FSSW). Finally, mechanical properties of joined materials such as tensile, tensile shear, bending and fatigue strength were tested. As a result, it has been verified that light metal alloys such as magnesium alloy AZ31B with good ductile and fatigue strength can easily be joined with FSSW. Moreover, joining of light metals with FSSW method has been demonstrated with comparative test and analysis in engineering applications.