GaN film grown on Si substrate was characterized by Rutherfordbackscattering/Channeling (RBS/C). The experimental results show that the thick-ness of GaN epilayer is about 2.5μm and the GaN film has a good crystalline quality(Xmin=3.3%). By using channeling angular scanning, the 0.35% of average tetragonaldistortion in GaN layer is observed. In addition, the depth profiles of strain in GaNfilm layer reveal that the strain in GaN film nonlinearly decreases with the increase offilm thickness. The strain-free thickness (above 2.5μm) of GaN film on Si substrate isfar below that (150μm) of GaN film on Sapphire.