Friction stir welding (FSW) has been so far the only feasible welding technique that can be applied for joining many hard to weld metals, especially high strength aluminum alloys. In this study, FSW of 6061-O aluminum alloys which are hard to weld by many fusion welding methods was investigated experimentally. For this purpose, FSW process was carried out with the combination of three different rotational speeds and one advancing speed to obtain the effect of heat generation on the welding quality. In this study, evaluating mechanical properties, including tensile properties and microhardness features of the friction stir welded (FSWed) plates are aimed. The main goal is to achieve sound weld that can provide high performance with regards to mechanical properties. The results demonstrate that the succesful FSW process can provide high performance in tensile strength, while reduces the elongation to failure. In addition, microhardness of the stir zones are observed to be higher than that of the base material for all FSWed joints.