Tensile properties and fracture location of AA2139-T351 friction stir welded joints are studied in the as-welded and post-weld aged condition. The experimental results show that when the joints are free of welding defects, they fail on the advancing side of the HAZ exhibiting a large amount of plastic deformation. When the revolutionary pitch exceeds a threshold value, some micro-defects are formed in the weld nugget due to insufficient heat input. In this case, the joints fail near the weld center, and the fracture occurs in a mixed mode, both ductile and brittle. However, being less ductile, post-weld aged joints are less defect-tolerant and, then, they fracture closer to the weld center, showing a reduced elongation at fracture and an UTS within the order of magnitude of the as-welded joints.