Abstract: The effects of pin tool offset on the internal quality and mechanical properties of the SR-FSWed joints was systematically studied. The results show that the nugget zone of the joint consisted of three layered structure areas and a wedge area without any trend distribution. A peculiar remaining defect at the interface occurred when the joint interface was located in the wedge area of the nugget zone at certain pin too offset. Mechanical properties results show that the tensile properties of the joints increased gradually with the pin offset from –6 mm to –3 mm, and all the joints were broken at the unconsumed interface defects. When the pin tool offset ranges from –2 mm to + 5 mm, the performance of the joints was optimal with the tensile strength 340 ~ 350 MPa, and the joints was all broken at the RS-TMAZ location. The performance of the joint started to decrease with + 6 mm pin offset.