The technology of epitaxy growth GaN/Si (111) with high temperature AlN buffer is investigated. The state of strain and crystalline quality of GaN epitaxial layer on Si(111) substrate is investigated by high resolution X-ray double crystal diffraction (HRXRD). The influence of the high temperature AlN buffer thickness on the surface morphologies of GaN films is characterized by the atomic force microscopy (AFM). The experimental results show that the Al pre-treatment time and the thickness of AlN buffer have a significant influence on the crystalline quality, state of strain and surface morphology of GaN. The optimal Al pre-treatment time is 10 s, and the thickness of AlN buffer is 40 nm. The good surface morphology of GaN epitaxial layer is obtained with the full width at half maximum (FWHM) of GaN (0002) of 452″, and (10-12) of 722″ by X-ray (XRD) double crystal diffraction.