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Increasing usage of magnesium and aluminum light metals in the transportation industry has made joining of these two metals one of the challenges for researchers and engineers. The aim of this study was to investigate the relationship between mechanical properties of lap friction stir welded Al-Mg plates and characteristics of the interface. Therefore, joining of aluminum and magnesium in various conditions were conducted. Optical and scanning electron microscopy analysis, micro-hardness test and tensile tests were performed on samples. The results showed that in the joints which Mg was on top, approximately 10 micron thick layer of intermetallic compounds is created, while in the Al-top joints, approximately 1 mm thick intermetallic compounds with solidified microstructure was visible. Mechanical test showed Mg-top joint have higher strength in comparison with Al-top joint. On the other hand hardness test of Mg-top joint showed more fluctuation than Al-top joint. Microstructural investigation also showed that in the Mg-top joint, formation mechanism of intermetallic compounds has occurred in solid state while in the Al-top joint, in addition to diffusion in solid state, eutectic formation in the molten state and solidification has occurred.