Nanoporous (NP) GaN thin films prepared with electrochemical etching method were investigated as photocatalysts in dye photodegradation systematically. The comparison of NP GaN thin films with GaN thin films showed that NP GaN thin films with high surface-to-volume ratio exhibited much better photocatalytic activity. In comparison with porous Si wafers, NP GaN thin films with lower surface area exhibited much better photocatalytic activity, because GaN is efficient not only for dye reduction like Si, but also for dye oxidation. Due to its ceramic-like chemical inertness, moreover, NP GaN showed more excellent stability to photodegrade organic dye than porous Si under basic conditions. The band gap of GaN can be modulated in visible-light region, which will be beneficial to a photodegradation system with concentrated solar light. (C) 2015 Elsevier Ltd. All rights reserved.