The microstructure and phase constitution near the diffusion bonding interface of Mg/Al dissimilar materials are studied using a scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The test results indicated that an obvious diffusion zone was formed near the Mg/Al interface during the vacuum diffusion bonding. The diffusion transition zone near the interface consists of various MgxAly phases. The transition region on the Mg side mainly consists of Mg crystals, and the new phase formed was the Mg3Al2 phase having a face-centred cubic lattice. This is favorable for improving the combined strength of Mg substrate and diffusion transition zone.