GaN:Er thin film has been prepared by direct current (DC) planar magnetron reactive sputtering deposition. The film was characterized by X-ray diffraction (XRD), ultraviolet—visible (UV—Vis) spectroscopy, transmission electron microscope (TEM) and photoluminescence (PL) spectroscopy, XRD shows the structure of the film was nano/poly-crystalline. The average grain size of the film was derived from XRD peaks using the Scherrer formula and the value was 58nm. TEM result indicated that the film was GaN nano-particles embed in amorphous matrix and the particle size was between 6—8nm. UV—Vis transmission spectrum shows that the transmission ratio of the film exceeds 80% in the visible zone from 500nm to 700nm. From UV-Vis spectrum, the optical band gap of the film was calculated using Tauc plot and the value was 322eV. Last, PL was measured by fluorescence spectrometer at room temperature and green emission of Er3+ ions was clearly observed for the transition: 4S3/2(2H11/2)—4I15/2(554nm).