Depth profiling measurements of photoluminescence on GaN epitaxial films grown on c-plane sapphire with metalorganic chemical vapor deposition have been performed. Dry etching technique of reactive ion etching is used with reactive gas of CCl2F2/H2/Ar under an operation power of 200 W. Before and after each etching, reflectivity and photoluminescence spectra are measured. Film thickness is determined from both the scanning electron microscopy and the interference oscillations of the reflectivity spectra. An excellent steady etching rate of 19.2 nm/min is established. The photoluminescence measurements show that both the near-band-edge and the yellow luminescence remain fairly constant until the film thickness of about 700 nm, and a large drop is obtained in the ratio of near-band-edge to yellow emission intensity under about 300 nm. Analysis shows that the yellow luminescence emitters are mostly confined within the near interface region, and supports the origin of yellow luminescence to be due to native defects instead of impurities.