Abstract: In order to investigate the effect of refill time on fracture mode and fracture mechanism on refill friction stir spot welding (RFSSW), the LY12 aluminum alloy plates with the thickness of 1.5 mm and 2.0 mm were welded and then the tensile-shear experiments were performed. Under shorter refill time, the atom diffusion effect at the interface between sleeve stir zone and thermo-mechanically affected zone is poor and the crack may appear near the top surface of upper plate, which results in the shear-plug fracture mode. When the refill time is long enough, the crack mainly propagates along the interface between the upper and lower plates, which causes the shear fracture of RFSSW joint. Moreover, microstructure, microhardness and fracture morphology can all verify the illustration about the fracture mechanism of RFSSW joint.