The properties of GaN∶Mg films grown by MOCVD on SiC substrate were studied using SEM, Raman scattering and photoluminescence spectra. The results indicate that some Mg atoms substitute Ga to become acceptors, while most of them exist as Mg interstitials(Mgi) and aggregate at defects and dislocations to reduce the tensile stress, hence a great deal of cracks were introduced during decreasing temperature process for inhomogeneous strain distribution. On the other hand, the incorporation of Mg aggravates disorder and debases the film quality. Finally, the room temperature photoluminescence measurements show the blue band attributed to DAP-type transitions from a deep donor(MgGaVN) to a shallow acceptor(MgGa).