An Al-4.5%Cu-1.5%Mg aluminum alloy with a T4 temper was friction stir welded, and the effect of the abnormal grain growth on the tensile strength of joints was investigated. Abnormal grain growth usually happens during post weld heat treatment. It is found that the tensile strength and elongation of the heat-treated joint will increase significantly if this phenomenon completely happens in stir zone. On the other hand stable grains in the stir zone have no effect on the mechanical properties of heat-treated joint.