The GaN thin film was grown on a 6-inch Si （111） substrate using a metal-organic chemical vapor deposition （MOCVD） system. The microstructure， surface morphology， and crystal quality of the GaN thin film were characterized by techniques such as spectroscopic ellipsometer， high-resolution X-ray diffraction， scanning electron microscopy， atomic force microscopy and transmission electron microscopy. The influence of variations in carrier gas flow rate during GaN growth on the uniformity and crystal quality of the film was investigated. The results indicate that as the H₂ flow rate increases， the precursor reaches the substrate surface more rapidly， enhancing the surface reaction and consequently increasing the growth rate of GaN. However， excessively high H₂ flow rates causes some of the mixed gases to participate in the GaN growth for insufficient time， leading to a saturation of the growth rate when the H₂ flow rate reaches 39 slm （standard liter per minute）. While increasing the H₂ flow rate also enhances the migration rate of Ga atoms， further increases up to 48 slm do not yield a smoother surface. The AlGaN buffer layer exhibits a V-shaped pit structure， with most dislocations being bent， annihilated， and prevented from extending into the GaN layer. This results in a growth process resembling lateral overgrowth， which improves the crystal quality of the GaN to some extent.