Abstract: The micro friction stir welding of overlap joints of ultra thin Al-alloy plate with thickness of 0.5mm were studied. In the micro friction stir welding process, zero tilt angle and the mini-shoulder friction stir tools were adopted. Especially the microstructure distribution and mechanical properties were analyzed in detail. The results show that the cone-shaped minishoulder friction stir tools with figures can reduce the width of weld seam because of the collecting effect of shoulder figures.The width of weld seam is only 3mm. This method provides a feasible solution for the welding and encapsulation of small thinwalled boxes. Due to the movement of welding tool in ultra high rotation speed, the overlap interface is stirred, mixed, broken, and better than the Hook interface of conventional FSW. The ratio of loading capability of the joint to base metal is 89.6%.