AlGaN/AlN/GaN　HEMT　structures　were　grown　on　sapphire　substrate　by　MOCVD　with　different　AlN　growing　time,　and　the　influence　of　AlN　thickness　on　electrical　properties　was　investigated.　When　AlN　growth　time　is　about　12　s　corresponding　to　the　AlN　thickness　of　1~1.5　nm,　the　sample　has　the　best　performance　of　electrical　properties　with　the　lowest　sheet　resistance　of　359　Ω·sq-1,　the　highest　2DEG　concentration　of　1.16×1013　cm-2,　and　a　high　2DEG　mobility　of　1　500　cm2·V-1·s-1.　AFM　results　indicate　that　AlN　layer　within　a　certain　thickness　range　has　little　influence　on　the　surface　morphology.　HRXRD　results　show　that　AlGaN/AlN/GaN　HEMT　has　a　good　heterostructure　interface.