We　apply　a　reverse　AC　electrical　stress　on　the　Gate　of　AlGaN/GaN　high　electron　mobility　transistors　(HEMTs).　The　change　of　Schottky　Barrier　height　(SBH)　with　respect　to　time　is　derived　from　the　I-V　and　C-V　characteristics,　respectively.　The　SBHs　calculated　from　the　two　methods　show　similar　decreasing　trend.　However,　the　one　from　I-V　method　recovers　partly　after　stress　is　removed.　The　behaviour　is　attributed　to　the　traps　in　AlGaN　barrier　and　surface　state　of　the　gate　as　a　result　of　the　electrical　stress,　and　the　recovery　of　surface　state　is　the　main　factor　for　the　recovery　phenomena.