The influence of Mg doping on structural and strain properties in GaN layers grown on sapphire substrates by metalorganic chemical vapor deposition was studied by means of high resolution X-ray diffraction and Raman scattering. The results showed that the disorder of GaN films aggravated and the quality reduced as Mg doping rate increasing. However, according to the theoretic calculation, the compressive stress determined by the Raman shift of the E2 mode was not due to the substitution of Mg atoms for Ga. Furthermore, the SEM measurements indicated that some Mg atoms substituted Ga to become acceptors, while most of them existed as Mg interstitials(Mgi) and aggregated at defects and dislocation, hence a great deal of cracks are introduced during decreasing temperature process for inhomogeneous strain distribution.