We experienced one case of immature teratoma at anterior mediastinum. the patient was Syears old female whose complain were cough and chest pain. Chest film showed mass density St the lower half of the Rt chest. Chest CT showed inhomogenous mass which have some calcified area. At the time of operation, 12x10x13cm sized mass have smooth surface and its stump elongated to the thymus. a-FP level, preoperative 22.5ng/ml, was decreased to 9.7ng/ml after operation. Postoperative adjuvant chemotherapy was performed with Vinblastin, Bleomycin and Cisplatin combination. The patient had an uneventful postoperative or postchemotherapy course and was discharged in a good condition.