Erbium has been diffused into GaN for the first time. A weak spontaneous emission is observed in the photoluminescence spectra after the diffusion process during 168 h at 800 °C under N2 atmosphere. The diffusion coefficient of erbium in GaN is obtained in Arrenhius expression to be D = 1.8 ± 1.3 × 10-12exp(-1 ± 0.4eV/kT) cm2/s. The result shows that the Er diffusion mechanism might be an interstitial-assisted process. The luminescence characteristics of the Er-diffused GaN is compared with the Er-implanted GaN. The methods to enhance the emission intensity of the Er-diffused GaN are discussed.