Effect of long anneals on densities of different types of threading dislocations (TDs) in GaN films grown onto sapphire substrate by metal-organic chemical vapor deposition was investigated by high-resolution X-ray diffraction. The results showed that the densities of both types of TDs changed obviously but oppositely, and residual stress in the GaN films was relaxed by generating edge-type TDs instead of screw-type TDs. The results obtained from chemical etching experiments and grazing-incidence X-ray diffraction (GIXRD) also supported the proposed defect structure evolution. (c) 2006 Elsevier B.V. All rights reserved.