Mn-doped GaN films (Ga1-xMnxN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga1-xMnxN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn3+ due to the thermal treatment. Mn3+, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn3+ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn3+ presents strongest ferromagnetic signal at annealing temperature 950 degrees C. Higher annealing temperature (such as 1150 degrees C) is not proper because of the second phase MnxGay formation. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.