GaN:Mn thin films were fabricated by implanting Mn ions into Mg-doped GaN epilayer. X-ray diffraction study reveals that all the samples are in single phase wurtzite crystalline structure. Raman spectra exhibit additional excitations attributed to the vibrational mode of defects caused by Mn ion implantation and the Mn related local vibrational mode in the vicinity of E-2(high). The X-ray photoelectron spectroscopy results indicate that the Mn ions were successfully incorporated into the GaN host lattice by substituting the Ga sites, The magnetization curve as a function of the magnetic field at 300 K indicates that room ferromagnetisms exist in GaN:Mn thin films. The effect of vacancy defect on the magnetic properties of Mn-Mg co-doped GaN has been studied by first principles calculations. Based upon experimental and theoretical results, it is believed that the alteration of ferromagnetism with annealing temperature is directly related to the variation of magnetic exchange interaction with microstructure in this material system. (C) 2016 Elsevier B.V. All rights reserved.