We synthesized ZnO nanowall networks on GaN/sapphire substrates by metal organic chemical vapor deposition without using metal catalysts. It was observed that the domain size of ZnO nanowalls decreased and the density of nanowall networks increased with increasing growth time. The ZnO nanowall networks were grown in a crystallized wurtzite structure and c-axis growth direction perpendicular to the substrate. The temperature dependent PL spectra of ZnO nanowall networks grown for a short growth time show stronger emission of longitudinal optical first phonon replicas of free-exciton than free-exciton emission as temperature increases in ZnO nanowalls, compared with those grown for longer growth time. It is interpreted that different PL spectra of ZnO nanowall networks are due to different Ga impurity diffusion into ZnO on GaN epilayer. (C) 2009 Elsevier B.V. All rights reserved.