The optical properties of nanoporous GaN formed by electrochemical etching were analyzed. Spectroscopic ellipsometry was used to evaluate the refractive index and extinction coefficient of nanoporous GaN and the results were compared with the properties of bulk GaN. The ellipsometric spectra were measured for samples fabricated using varied etching conditions and they were fitted to obtain the optical parameters. The refractive index of nanoporous GaN decreased with porosity and it was as low as 1.97 at the wavelength of 400 nm. In addition, a deeply etched nanoporous layer showed an enhanced extinction coefficient in the below band gap region. The measured optical properties of nanoporous GaN will be of great importance for designing an optical device based on an index-controlled nanoporous layer. (C) 2012 Elsevier B.V. All rights reserved.