GaN thin films are grown on patternted 2 inch (5.08 cm) Si(111) substrates by metal organic vapour phase epitaxy (MOVPE). AlN interlayers with different thicknesses are introduced between the compostion-graded AlGaN buffer layers and the GaN seed layer in different samples, and the influence of AlN interlayer on the growth of GaN film is investigated. The results indicate that the full widths at half maximum (FWHMs) of (002) and (102) X-ray diffraction (XRD) rocking curves as well as the crack density are improved obviously with increasing AlN interlayer thickness. The AlN interlayer can change the growth mode of GaN seed layer. GaN seed layer tends to grow in islands mode with a thicker AlN interlayer. This leads to epitaxial lateral overgrown of subsequent n-GaN, which can decrease the density of dislocations and the residual tensile stress of GaN film. Besides, a new method for studying the morphology and growth mode of GaN seed layer by observing yellow luminescence using fluorescence microscope is presented.