We have investigated the effects of N + -implanted sapphire (0001) substrate on a GaN epilayer grown by metal-organic chemical vapor deposition (MOCVD). As a result of implantation with 55keV nitrogen ions (N + ) to a dose ranging from 1x10 1 5 to 1x10 1 7 cm - 2 prior to GaN epilayer growth, the N + -implanted sapphire surface was chemo-physically modified and a thin disordered AlN phase was observed. The N + -implanted substrate's surface had decreased internal free energies during the growth of the GaN epilayer, and the misfit strain was relieved through the formation of an AlN phase on the N + -implanted sapphire (0001) substrate. Crystallographical and optical properties of GaN epilayer grown on N + -implanted sapphire (0001) substrate with the ion dose of 1x10 1 6 cm - 2 were found to be improved, indicating decreased internal stress and generation of dislocations in the GaN epilayer. The present results show that N + -implantation pre-treatment of the sapphire (0001) substrate surface can be used to improve the properties of GaN epilayers grown by MOCVD.