The benign teratoma is usually slow growing tumor, but we expirienced a case of primary huge mediastinal benign teratoma that had grown very rapidly, maximally during 3 years. The 14-year-old female patient was admitted to our hospital because of abnormal chest X-ray that showed 10$ imes$10cm sized well definded mass with multiple calcificactions. but the mass was not present in chest X-ray perfomed on 3 years prior to admission. Under the diagnosis of teratoma, complete surgical resection was done by the left thoracotomy. The result of pathology was benign teratoma.