The characteristic features of myasthenia gravis, which are weakness of voluntary muscle and easy fatigability, result from defective neuromuscular transmission caused by an autoimmune response to acetylcholine receptor. Recently, we performed two cases of thymectomy for the treatment of myasthenia gravis, one was 50 year old man who had malignant thymoma and the other was 19 year old girl who had benign thymic hyperplasia. The former had long duration of symptoms, and showed poor postoperative result. The latter had short duration of symptoms, and showed complete remission. Herewith, we suggest that early thymectomy can give the clinical assets for myasthenia gravis. So we report these two experiences with review of literatures.