The micro friction stir welding (μ-FSW) of lap joint of thin Al-alloy plate with a thickness of 0.5mm and SiC reinforced Al composite were investigated, concentrating on the process characteristics, macro-morphology of weld cross-section and mechanical properties of the joints. The mechanisms of migration interface morphology formation and joint failure were revealed. The results show that with suitable indentation depth, wels of good appearance can be obtained in a wide range of process window. The weld cross-section shows a special “Hoop shape” migration interface morphology, caused by the characteristics of μ-FSW process and SiC particles. The joint failure mode is the tensile fracture of “thinning area” at the upper rim of migration interface. And the joint mechanical properties are directly related to the loading direction, which is caused by the different metal plastic flow behaviors under different loading directions