Abstract: The objective of this paperwas to represent the influences of processing parameters (rotation speed, radius of turn)of com-stir friction stir spot welding on the mechanical properties of welded joints by using LF21 aluminum alloy as base material.According to the results, the mechanical properties of welded joints increase along with the rotation speed of the tool as other welding parameter is constant.Shearing force of welded joints reaches as high as 3.47 kN when the rotation speed of the tool is 1200 r min.The mechanical properties of welded joint worsen when the rotation speed of the tool is higher than 1200 r min.When radius of turn only by changed, the mechanical properties of welded joints increase along with the radius of turn, and shearing force reaches 3.47 kN when the radius of turn is 0.5 mm.The mechanical properties of welded joints worsen when the radius of turn is over 0.5 mm.When LF21 aluminum alloy was used as base mateial, the differences of the microstructure between com-stir friction stir spot welded joints and friction stir spot welding joint are that the microstructure of com-stir friction stir spot welding joint has the "ear zone" formed by second-layer plasticity ring besides the original plasticity ring.