Abstract The characteristics of GaN films diffused with Mg were studied. The undoped GaN films were grown by metalorganic chemical vapor deposition (MOCVD). The photoluminescence (PL) spectra of Mg diffused GaN showed a broad violet emission. The Mg-diffused GaN was p-type conductivity with a mobility of 13 cm 2/V-s and a hole concentration of 3×10 15/cm 3 at a diffusion temperature of 1100°C. However, the samples, which were diffused at 1100°C, showed a red emission peak after the annealing process. From the diffusion depth profile observed by secondary ion mass spectrometry (SIMS), we obtained the activation energy of 1.3 eV for Mg diffusion in GaN.