Abstract Thermal stabilities of GaN epitaxial layer and of GaN/sapphire interface has been investigated using Raman scattering, photoluminescence (PL), optical transmission and SEM analysis. GaN epitaxial layers were annealed up to temperatures as high as 1100°C in nitrogen ambient for a period of 20 min. Raman scattering identifies two new additional vibrational modes in GaN between E 2 and A l(LO) modes for the samples annealed beyond 900°C. Annealing up to the temperature of 900°C enhances the PL intensity and it is decreased drastically for the high-temperature annealed samples. Optical transmission and SEM studies exhibit a clear indication of reaction between GaN/sapphire interface at higher temperatures above 1000°C.