AlN　buffer　and　GaN　epitaxial　layer　were　prepared　by　MOCVD　on　Si(111)　substrate.　The　effect　of　H2　carrier　gas　flow　for　AlN　buffer　epitaxy　on　GaN　was　investigated　by　high　resolution　X-ray　diffraction,　ellipsometer　and　atomic　force　microscope.　It　is　found　that　AlN　thickness　increases　(i.e.　the　increasing　of　AlN　growth　rate)　with　the　increasing　of　H2　flow.　The　surface　roughness　of　AlN　also　tends　to　increase.　The　change　in　surface　roughness　is　attributed　to　the　enhancement　of　island-growth　mode.　The　increasing　of　AlN　buffer　thickness　contributes　to　the　increasing　of　tensile　stress　which　promotes　AlN　island　growth　mode.　The　higher　density　of　islands　with　bad　orientation　was　observed　by　AFM　on　AlN　buffer　layer　which　was　grown　with　higher　H2　flow.　ω　scan　of　(0002)　and　(101¯2)　show　that　the　increasing　of　H2　flow　leads　to　the　increasing　in　FWHM　of　GaN(i.e.　the　increasing　in　density　of　screw　threading　dislocation　and　edge　threading　dislocation).　Because　the　three-dimensional　growth　of　GaN　starts　on　the　top　of　AlN　islands,　the　AlN　buffer　layer　with　high　density　of　islands　contributes　to　rapid　coalescence　of　GaN　islands　that　will　lead　high　density　of　edge　threading　dislocation.　The　bad　orientation　of　AlN　islands　on　buffer　layer　will　lead　to　GaN　thin　film　with　high　density　of　screw　threading　dislocation.　The　obtained　data　demonstrate　that　the　H2　carrier　gas　flow　plays　an　important　role　in　improving　the　crystal　quality　of　GaN.