Photoluminescence depth profiling of highly strained In 0.1 Ga 0.9 N and In 0.15 Ga 0.85 N epitaxial films have been studied employing an inductively coupled Cl 2 plasma etching. The photoluminescence measurements showed that thick InGaN films (0.2 µm) consist of three different structural phases; (i) an InN-rich region near the InGaN film surface, (ii) a region that was free from InN-rich phase under stress-relaxation in the middle of the film, and (iii) an InGaN/GaN interface region. In region (i), a higher wavelength peak from the InN-rich phase was dominant. After removing the surface layer of 500 Å, the PL peaks from InN-rich phases completely disappeared, suggesting that the InN-rich phase region is confined to a depth of 500 Å. In regions of (ii) and (iii), the strain-relaxation between InGaN and GaN had a significant influence on the luminescence properties of InGaN.