The interface microstructure, formation of diffusion bonded joint and regulation of atom diffusion were studied by means of scanning electron microscope (SEM), energy dispersion spectroscopy (EDS) and electron probe microanalyser (EPMA). The experimental results indicated that an obvious interfacial transition zone was formed between Mg and Al, and there are three intermetallic layers Mg17Al12, MgAl and Mg2Al3 in this zone. Diffusion activation energy of Mg and Al in the above layers was lower than that in the Mg and Al base metals. The thickness (x) of each layer can be expressed as x(2) = 4.14exp(-28780/RT)(t-t(0)), x(2) = 31.4exp(-25550/RT)(t-t(0)) and x(2) = 0.6exp(-22600/RT)(t-t(0)) corresponding to Mg17Al12, MgAl and Mg2Al3 with heating temperature (7) and holding time (t).