Monolithic AlN，BN films and AlN/BN multilayers were prepared by reactive magnetic sputtering. The films were characterized by X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation. Results showed that the w-AlN phase and amorphous BN films structure exist in AlN and BN monolithic films. The crystal structure of BN layers in the multilayers depends on the thickness of BN layers. When the thickness of BN is less than 0.55nm, BN layers grew epitaxially with AlN under the influence of the “template effect" of w-AlN layers and form the same structure as AlN. When the thickness of BN is larger than 0.74nm, BN layers are amorphous. The hardness of AlN/BN multilayers also depends on the thickness of BN layers. When the thickness of BN is 1—2 molecular mono_layers, the superhardness enhancement of the multilayer is observed. The superhardness effect disappears when the thickness of BN is larger than 0.74nm. The critical thickness of BN layers marking the change from the crystalline to the amorphous structure has been calculated. The hardening mechanism of AlN/BN multilayers has been discussed. The main reason of the superhardness enhancement of the multilayer is the stress and modulus difference between the layers of BN and AlN.