Both the electronic structures and the optical properties of single-walled zigzag GaN nanotubes (NTs) with Mg-Ga-O-N co-doping are investigated using first-principles calculations. We find that the Mg-Ga-O-N defect complex can exist stably in GaN NTs. The direct band gap width of the GaN NTs can be reduced by means of the Mg-Ga-O-N co-doping. The electrons of the valence band maximum (VBM) state are localized around the N atoms bonded with the Mg atom. The imaginary part epsilon(2) of the complex dielectric function of GaN NTs with Mg-Ga-O-N co-doping has a sharp peak closely related to the optical transitions between the VBM and conduction band minimum states. (C) 2012 Elsevier B.V. All rights reserved.