GaN　thin　film　with　AlN　nucleation　layer　was　grown　on　Si(111)　substrates　using　metal-organic　chemical　vapor　deposition　(MOCVD).　High　resolution　X-ray　diffraction　(XRD),　ellipsometer　and　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　layers,　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　strain　exists　on　all　GaN　samples　in　c-axis　direction　and　reduces　slightly　with　the　increasing　of　AlN　thickness.　ω　scan　shows　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　increasing.　The　change　in　dislocation　density　is　attributed　to　the　different　grain　sizes　and　grain　densities　on　AlN　nucleation　layers.　The　crystal　quality　of　GaN　epilayer　deposited　on　AlN　nucleation　layer　with　large　grain　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　formation　of　dislocations.　These　dislocations　degrade　the　crystal　quality　of　GaN.