Ab initio calculations based on density functional theory are performed to study the stability of newly proposed C3N4 forms. Heptazine-based g-C3N4 was found to be energetically favored relative to other phases. The quasiparticle band energies of different C3N4 phases are calculated using the GW method. Among the seven phases of C3N4 studied, only the pseudocubic phase and g-h-triazine phase have direct band gaps, and all of the other phases have indirect band gaps. The band gap of alpha-C3N4, beta-C3N4, cubic-C3N4, pseudocubic-C3N4, g-h-triazine, g-o-triazine and g-h-heptazine is 5.49 eV, 4.85 eV, 4.30 eV, 4.13 eV, 2.97 eV, 0.93 eV and 2.88 eV, respectively. From the viewpoint of band gap energies, both the g-h-heptazine and the g-h-triazine phases can be used as suitable photocatalysts for hydrogen production using water. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.