The thermal stability of cubic-phase GaN (c-GaN) films are investigated by photoluminescence (PL) and Raman scattering spectroscopy. C-GaN films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition. PL measurements show that the near-band-edge emissions in the as-grown GaN layers and thermally treated samples are mainly from c-GaN. No degradation of the optical qualities is observed after thermal annealing. Raman scattering spectroscopy shows that the intensity of the E2 peak from hexagonal GaN grains increases with annealing temperature for the samples with poor crystal quality, while thermal annealing up to 1000 °C has no obvious effect on the samples with high crystal quality. © 1999 American Institute of Physics.