Previous studies showed that reactive ion beam (RIB) pretreatment of sapphire prior to GaN deposition results in the reduction of dislocation density in the GaN film. It was also found that an amorphous phase remained at the interface region after the GaN deposition at high temperature. Annealing was performed to obtain the structural change due to the recrystallization of the remaining amorphous phase, and the effect on the electrical properties of the GaN thin film on RIB treated sapphire (0001) substrate. DCXRD spectra and Hall mobility of the specimen were studied as a function of the annealing time at 1000 C in N 2 atmosphere. For the annealed specimen, FWHM of DCXRD decreased and the mobility increased. The annealed specimen was compared with a not annealed sample by TEM. A decrease of lattice strain and a reduction of the dislocation density about 56-59% was observed. The present results clearly show that the combination of RIB pretreatment and proper post annealing conditions improve the properties of GaN films grown by MOCVD.