GaN thin films have been grown on Si(111) substrates by metal-organic chemical-vapour deposition. The analyses of X-ray diffraction indicate that GaN is of wurtzite structure with the 〈0001〉 crystal orientation. Raman spectroscopy is used to study the sample under different pressures. The results of Raman spectroscopy,show that when the sample is under relaxation,GaN flims are in tensile stress the maximum of which is 342.272 MPa and Si substrates are in compressive stress. But due to the existence of a buffer layer between GaN flims and Si substrates,the values of tensile and compressive stress are not identical. After applying different pressures parallel to the (0002) plane of GaN,the peaks of GaN and Si are both blue shit as the pressures increasing,which supports that applied pressures lead to compressive stress in the interior of the sample.