Abstract: Dissimilar aluminum alloys of 5052-H112 and 2024-T4 (2 mm) were successfully joined by refill friction stir spot welding. Defects-free joint was obtained under optimum welding conditions. However, the welding parameters window was rather narrow. At the high heat input, lots of defects such as ligament bonding, voids, cracks and kissing bonding appear, which accumulated at the interface and the path in which the sleeve plunges into the workpiece. The mechanical properties tests show that the preferable strength is obtained under lower heat input welding conditions, which is consistent with that the defects-free joint tends to form under lower heat input welding conditions. The failure of the joint occurred at the interface and the path where the sleeve plunges into the workpiece both under tensile and cross-tension loadings. The effective metallurgical bonding formed at the interface.