Quality properties and internal defects of unintentionally doped GaN films grown on 0.3 vicinal sapphire (0001) substrates by MOCVD are investigated by TEM. The results show that plenty of dislocations in the GaN films prepared on vicinal sapphire substrates are annihilated in the areas with a distance of 0.8 m away from substrates, and that dislocations gather in the GaN films. Based on these phenomena, a mechanism for dislocation annihilation in the GaN film prepared on vicinal substrate is proposed, which is capable of explaining the fact that vicinal substrates are able to improve the qualities of GaN films.