Using DC faced target sputtering method we grow AIN the films on the $Al_2O_3$(0001) substrate with varying thickness(17$AA$-1000$AA$). We measured x-ray diffraction(XRD) profiles by synchrotron radiation($lambda$=1.12839 $AA$) with four circle diffractometer. The full width half maximum(FWHM) of rocking curve for the AIN (0002) diffraction of the film grown at $500^{circ}C$ was $0.029^{circ}$. Also, we confirmed that the stress between AIN thin film and $Al_2O_3$(0001) substrate was reduced as increasing AIN film thickness, and the critical thickness of 400~500 $AA$, defined as a lattice constant in the film agrees with that in a bulk without stress, was obtained.