The density of the vacancy-type defect in Er doped GaN was measured by positron annihilation spectrometry (PAS) and the correlation between the intensity of the Er-related luminescence was studied. A luminescence peak at 558 nm originating from S-4(3/2) to I-4(5/2) transition of Er3+ was observed in Er-doped GaN. The intensity of the luminescence increased with increasing Er concentration and showed the maximum with the Er concentration of around 4.0 at%. The PAS measurements showed that the vacancy-type defect density increased with increasing Er concentration up to 4 at%, and around 4 at% of Er, the formation of defect complex such as V-Ga-V-N was suggested. The contribution of the defect to the radiative recombination of intra-4f transition of Er is discussed. (C) 2009 Elsevier B.V. All rights reserved.