The present investigation is aimed at evaluating the influence of tool rotation rate and welding speed on the microstructure, tensile properties, and fracture mode of 6061 Al-T651 alloy after friction stir welding (FSW). TEM results revealed that in the nugget zone (NZ), FSW resulted in the dissolution of fine needle-shaped precipitates that previously existed in the base metal. At a given rotation rate of 1400rpm, the yield strength (YS) and ultimate tensile strength (UTS) of the welded joints increased with increasing welding speed from 200 to 600mm/min. However, the UTS of the joints was nearly independent of the rotation rate. Furthermore, the relationship between the hardness distribution and fracture location has also been identified.