The epitaxial AIN thin films on AI2O3(0001) were prepared byDCfaced-target sputtering. Synchrotron x-ray diffraction was employed to measure the lattice constants a and c of AIN films over a thickness range of 20 Ä to 1 μ m, and the lattice constant as a function of thickness was fitted by the equlibrium theory. From the fitting result, the critical thickness of AIN on AI2O3(0001) to be estimated as 14.4 × 1.5 Ä. The surface oxidation of AIN film was confirmed by electron spectroscopy for chemical analysis(ESCA) profile. We confirmed that the surface oxidation layer thickness was unchanged and the thickness of oxidation layer was estimated about 20–40 Ä.