The mechanism of dislocation formed and strain released in GaN epitaxial layer grown on sapphire substrates by molecular beam epitaxy (MBE) was investigated by comparative analysis of the in-situ spectroscopic ellipsometry (SE) data. It has been found that the dislocation density was related to the extent of strain relaxation in GaN film grown on just sapphire substrate during buffer layer ramping and that threading and spiral dislocation was suppressed effectively by using vicinal sapphire substrates. The results indicate that using vicinal sapphire substrates is a practicable way to suppress spiral dislocation and to improve crystal structure of GaN films.