The strain in GaN epitaxial layers grown on 611-SiC substrates with an AIN buffer by metalorganic chemical vapor deposition is investigated.It is found that the insertion of a graded AlGaN layer between the GaN layer and the AIN buffer can change the signs of strain.A compressive strain in an overgrown thick(2μm)GaN layer is obtained.High-resolution x-ray diffraction, Raman spectroscopy and photoluminescence measurements are used to determine the strain state in the GaN layers.The mechanism of stress control by inserting graded AlGaN in subsequent GaN layers is discussed briefly.