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 58nm. 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 322eV. 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).