An undoped a-plane GaN epi-layer was grown on r-plane sapphire substrate with silica nano-spheres by metal organic chemical vapor deposition. Defect states in the GaN epi-layer with Pt Schottky diode and Ti/Au Ohmic contact were characterized by using a deep level transient spectroscopy (DLTS) measurement. According to the results of DLTS spectra for a-plane GaN epi-layer with silica nano-spheres, the defect state with activation energy of 0.56 eV and capture cross section of 9.72 x 10(-16) cm(-2) originated in non-interacting point defect appeared dominantly. A dislocation related defect also appeared with small intensity. It shows that the silica nano-sphere layer integrated in the valley of the buffer layers on the r-plane sapphire substrate can improve the electrical property by the reduction of defect states in the GaN epi-layer. (C) 2012 Elsevier B.V. All rights reserved.