Abstract: Through analyzing the microstructure and testing mechanical properties of friction stir welded Al-Li alloy lap joints,the effect of FSW welding parameters on the tensile properties of the lap joints was investigated. The results showed that the length of stir pin has significant influence on the tensile properties of lap joints. When the length of pin is changed from 2.8 mm to 2.5 mm,the ultimate strength and elongation of lap joints are obviously improved. Furthermore,the rotation speed/welding speed (η) also affected the performance of joints. When there was a small increasing in η,the tensile properties of lap joints increased accordingly. For the Al-Li alloy lap structure, the optimum tool rotational speed is 800 r/min,welding speed is 200 mm/min and the length of pin is 2.5 mm,the ultimate strength of lap joint reaches 467 MPa,which is equivalent to 94% that of the base metal and the elongation is 3.18%. In addition, the analysis of tensile fracture appearance indicated that the tensile cracks initiate from the location of "Hooking" defect in the advancing side and grow along the HAZ until to the base metal.