Abstract: The microstructure and lap shear properties of refill friction stir spot welded dissimilar 7075/6061 aluminum alloys joints were studied. 7075 aluminum alloy was used as the upper sheet. The effect of sleeve plunge depth on joint formation and mechanical properties were discussed. The results showed that defect-free joints were obtained when plunge depth was small that the upper sheet was just penetrated. Defects such as void, tear and incomplete refilling appeared in the weld as the sleeve plunge depth increased. Hook and lap interface at joint center, which is critical to the mechanical properties of the joint, was well jointed when shoulder plunge depth was small. The fracturing load was firstly increased and then decreased with the increase of sleeve plunge depth. The maximum fracturing load was 7 236 N when using 3.4 mm.