Abstract: The structure and crystal quality of GaN are studied using Rutherford backscattering spectrometry (RBS) and channeling. The samples are grown on sapphire substrate by metalorganic vaporphase epitaxy. The He+, N+ implantation with different ion energy and postimplantation annealing are investigated. 7～8 orders increasing of resistivity is observed by Hall measurements after specific temperature annealing, and the optimized annealing temperature is about 200 and 400 ℃ for He+ and N+, respectively. After 600～700 ℃ annealing, the resistivity is still very high, and radioation damages is found by RBS/channeling.