Optical characteristics of Mg-ion implanted GaN layers with various fluence ranges were evaluated. Mg ion implantation was performed twice at energies of 30 and 60keV on n-GaN layers. The first implantation at 30keV was performed with three different fluence ranges of 1.0×1014, 1.0×1015 and 5.0×1015cm−2. The second implantation at an energy of 60keV was performed with a fluence of 6.5×1013cm−2. After implantation, samples were annealed at 1250°C for 1min under N2 atmosphere. Photoluminescence (PL) spectrum of the GaN layer with the Mg ion implantation at the fluence range of 1.0×1014cm−2 at 30keV was similar to the one of Mg-doped p-GaN layers grown by MOVPE (Metal-Organic Vapor Phase Epitaxy) on free-standing GaN substrates and those at the fluence ranges over 1.0×1015cm−2 were largely degraded.