The relationship between optical and structural properties of Er-doped GaN grown by molecular beam epitaxy on sapphire (0 0 0 1) substrates was studied. The Er-related luminescence with wavelength at around 558 nm originated from S-4(3/2) to I-4(15/2) transition was observed. The intensity of the Er-related luminescence increased abruptly with the Er concentration of about 2 at% and showed a maximum at about 4at%. X-ray diffraction analyses and Rutherford backscattering measurements suggested that most of the Er ions are incorporated into substitutional lattice site with the Er concentration of up to 2 at%, and then begin to occupy interstitial sites. The incorporation site of Er was found to be dependent on the Er concentration. Based on these results, the optically active center of Er in GaN is discussed. (C) 2008 Elsevier B.V. All rights reserved.