Abstract: In view of the unavoidable defects in casting structure, friction stir welding (FSW) was performed to repair ZL210 casting Al alloy. Effects of rotational speeds on microstructures and mechanical properties were mainly investigated. The experimental results showed that the cavity defect appeared in stir zone (SZ). The defect-free SZ could be attained under reasonable welding parameters and was featured by fine and equiaxed grains. The hardness distribution of repaired region presented W-shape, and the width of heat affected zone (HAZ) increased with the increasing of the rotational speed. In addition, with the increase of rotational speed, the tensile properties of repaired region firstly increased and then decreased. The maximum tensile strength and elongation of repaired region were attained at 1 500 r/min and the values were 318 MPa and 11.8% respectively. The fracture surface morphologies showed the ductile fracture of repaired region.