The effects of rotational speed on the intermetallic compounds and low melting point eutectic of Al/Mg friction stir welded joints were investigated. The interfacial microstructures of both Al and Mg sides were characterized by electron backscatter diffraction. The results show that when the low rotational speed of 375 r/min is used, the eutectic layer (Mg+Al₁₂Mg₁₇), with an average thickness of 38.83 μm, appears at the upper of the Mg side interface. A continuous columnar Al₃Mg₂ layer with a thickness of 12.3 μm, perpendicular to the boundary between the Al₃Mg₂ layer and eutectic layer, is also found at the upper of the Mg side interface. At the middle and bottom of the Mg side interface, there are merely Al₃Mg₂ layer and Al₁₂Mg₁₇ layer, the thickness of which decreases sequentially from the upper to the bottom along the thickness direction. In addition, the Al₃Mg₂ layer with high kernel average misorientation at the Al and Mg side interface provides a path for the diffusion of Al and Mg atoms. When the high rotational speed of 600 r/min is used, the eutectic layer composed of Mg solid solution and Al₁₂Mg₁₇ phase is distributed across the Mg side interface along the thickness direction, and the thickness of the eutectic layer increases significantly compared with that at the low rotational speed of 375 r/min. The average thickness of the Al₃Mg₂ layer and eutectic layer on the upper of the Mg side interface is 32.89 and 68.92 μm, respectively. Finally, the strain rate caused by rotational speed plays an important role in the growth of intermetallic compounds.