GaN　epitaxial　films　with　different　thickness　of　AlN　buffer　layers　were　grown　on　Si　(111)　by　metal-organic　chemical　vapor　deposition　(MOCVD).　The　effects　of　AlN　buffer　thickness　on　in-plane　stress,　surface　morphology　and　crystal　quality　of　GaN　materials　were　investigated.　It　is　observed　that　the　AlN　buffer　with　thickness　of　15　nm　can　effectively　suppress　the　diffusion　of　Si　from　the　substrate.　Moreover,　the　AlN　buffer　can　also　introduce　a　relatively　large　compressive　stress　to　GaN　layers　and　eliminate　the　crack.　The　crack-free　and　mirror-like　GaN　layer　was　obtained　by　depositing　on　the　AlN　buffer　layer　with　thickness　of　15　nm.　The　full　width　at　half　maximum　of　the　film　is　as　low　as　536　arcsec　for　(0002)　and　594　arcsec　for　(101¯2)　reflection,　the　in-plane　tension　stress　is　0.3　GPa,　and　the　RMS　roughness　tested　by　atomic　force　microscopy　is　0.2　nm.