Abstract: AlCN amorphous thin films with different Al contents were deposited on Si(100) substrates by magnetron sputtering, followed by vacuum annealing at 700 ℃ and 1 000 ℃ respectively.The microstructures of the as-deposited and annealed films were characterized by X-ray diffraction(XRD) and high-resolution transmission electron microscopy(HRTEM), and the values of nanohardness and elastic modulus were measured by the nano-indention method.The results indicated that the organization and microstructure of the annealed AlCN films strongly depended on Al content.For the thin films with low Al content, delamination rather than crystallization occurred after annealing at 1 000 ℃, but for the thin films with high Al content, annealing accelerated the formation of AlN nanocrystallites, which were embedded into amorphous matrices.The density and size of the nanocrystallites were both found decreasing with increasing depth from the film surface.With the increasing of annealing temperature, both the values of nanohardness and elastic modulus decreased;but for the AlCN films with high Al content the decreasing trends slowed down due to the formation of nanocomposite structure.