A relationship between intra-4f transitions of Er and vacancy-type defects in Er-doped GaN was studied by using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Er-doped GaN grown by molecular beam epitaxy. A clear correlation between the defect concentration and the photoluminescence ͑PL͒ intensity was observed. The major defect species detected by positrons was identified as a Ga vacancy V Ga , and its concentration increased with increasing Er concentration ͓Er͔. For the sample with ͓Er͔ = 3.3 at. %, the maximum integrated intensity of PL was observed. The V Ga concentration was above 10 18 cm −3 and additional vacancies such as divacancies started to be introduced at this Er concentration. For the sample with higher ͓Er͔, the PL intensity decreased, and the mean size of vacancies decreased due to an introduction of precipitates and/or metastable phases.