Graves disease occur in association with myasthenia gravis is rare. We report a case of Graves disease and myasthenia gravis treated by bilateral subtotal thyroidectomy and total thymectomy simultaneously. A 37 year old woman was admitted with anterior neck mass and ptosis. Various examinations were compatible with combined Graves disease and myasthenia gravis. The bilateral subtotal thyroidectomy and total thymectomy were done simultaneously. The pathologic diagnosis was Graves disease and thymic hyperplasia. The patients postoperative course was uneventful. The thyroid function of patient became euthyroid and the clinical symptoms related with myastenia gravis resolved during follow up period.