The 3 mm thickness industrial pure aluminum and AZ31 magnesium alloy dissimilar metals joints were successfully manufactured by friction stir welding（FSW）. The influence of post-weld annealing treatment on microstructures and mechanical properties of dissimilar Al-Mg friction stir welded joint were investigated in detail. Using OM

EDS

IMCs deformability testing

tensile properties testing and SEM for fracture surface analysis. The results showed that the interface IMCs have obvious growth after annealing treatment. The interface IMCs layer cannot cooperate deformation with the both sides of the matrix metal

almost no deformation ability;the tensile strength of the as-welded FSW Al-Mg dissimilar joints was 106 MPa

92% of the base metal（BM）of aluminum.Meanwhile

the elongation was 3.2%

91% of BM of Al. The tensile strength and elongation of the Al-Mg joints decreased with the IMC grown up during the process of thermal of 300 ℃ with the holding time of 20 min

40 min and 60 min

which were 94 MPa/1.3%

70 MPa/0.7% and 28 MPa/0.4%

respectively. Accordingly

the fracture mode changed from ductile fracture to brittle fracture.