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