Abstract: A pure Al film was sputtered by DC magnetron on AZ31B magnesium alloy and heat-treated in a high vacuum. It was determined by the analyses of SEM and XRD that the Al film was ultra-fine polycrystalline. The thickness, critical adhesive force, hardness and elastic modulus of the Al film were measured by nano-indentation/scrateh test; the composition profile and properties were tested by glow discharge spectrometry (GDS). The results show that the thickness of the Al film decreases with increasing heat treatment temperature. Compared with the magnesium alloy, the Al film has a higher surface hardness and elastic modulus which decrease with increasing depth. A transition nano-scale layer forms between the Al film and substrate, which is of adherent trait and reveals somewhat elastoplastic property. It is concluded that the magnesium alloy can be protected by the Al film.