Myasthenia gravis with thymoma is associated with an increased risk of second malignancy, but concurrence of myasthenia gravis and thyroid carcinoma is rarely seen. In the treatment, it is emphasized to operate in two stage to avoid myasthenic crisis after surgery. In general, a thyroid cancer surgery is performed after controlling yasthenia gravis by means of thymectomy or medical treatment with steroids and anticholine esterase. We experienced two cases of thyroid carcinoma coexisting with myasthenia gravis, which is thought to be true first report in Korea.