We report here a case of myasthenia gravis complicated with hyperthyroidism and thymic hyperplasia. The patient was a 13-year-old girl with struma and hyperthyroidism which began at age 12. Two weeks following the initiation of treatment against hyperthyroidism she developed left blepharoptosis, diplopia, and dysphagia, which responded promptly to edrophonium administration. An increase of the anti-acetylcholine receptor antibody was found in the serum. A chest CT showed a large soft tissue mass in front of the ascending aorta, which was proven histopathologically as thymic hyperplasia. The patient underwent an extensive thymectomy and was placed on combination therapy with an anti-thyroid drug, glucocorticosteroid, and an anti-cholinesterase drug. Her symptoms and signs have been well controlled by this treatment. Coexistence of myasthenia gravis, hyperthyroidism, and thymic hyperplasia in childhood have never been documented in literature.