GaN thin film with AlN nucleation layer was grown on Si(111) substrate s using metal-organic chemical vapor deposition (MOCVD).High resolution X-ray diffraction (XRD),ellipsometer a nd atomic force microscope (AFM) are used to investigate the effect of AlN thickness on GaN film.The results demonstrate that the thickness of low temperature AlN nucleation layer prominently influences the morphology of AlN nucleation lay ers,the crystal quality and stress of GaN epilayers.It is found that the surface roughness of AlN samples is larger with thicker AlN nucleation layer.θ－2θ scan of (0002) shows that the compressive stra in exists on all GaN samples in c-axis direction and reduces slightly with the increasing of AlN thickness.ω scan sh ows that FWHM of (0002) increases while FWHM of (10-12) decreases with the increasing of AlN thickness.FWHM of (10-12) is related with the density of edge-type threading dislocation (ETD).It means that ETD reduces with the AlN thickness in creasing.The change in dislocation density is attributed to the different grain sizes and grain densiti es on AlN nucleation layers.The crystal quality of GaN epilayer deposited on AlN nucleation layer with large gra in size and low grain density is good,since the lateral growth and coalescence of GaN islands are promoted.On the other hand,the AlN nucleation layer with small grain size and high grain density leads to format ion of dislocations.These dislocations degrade the crystal quality o f GaN.