The structure and crystal quality of GaN using Rutherford Backscattering Spectrometry and Channeling were studied. The samples were grown on sapphire by metal-organic vapor-phase epitaxy. The H+, He+, N+ with different ion energies and doses were implanted into GaN. Post-implantation annealing were investigated. We observed 7-8 orders increasing of resistivity by Hall measurements after specific temperature annealing, and the optimized annealing temperature is approximately 200-400 degreesC for H+, He+ and N+ respectively. Even after 600-700 degreesC annealing, the resistivity is still very high. We think vacancies and radioactive damage by implantation are responsible for resistivity changes in GaN samples. (C) 2002 Elsevier Science B.V. All rights reserved.