Primary mediastinal immature teratoma is a rare germinal tumor which includes various elements of mature teratoma, choriocarcinoma, yolk sac carcinoma, embryonal carcinoma, and seminoma in some proportions. The tumor is virtually restricted to young man and the response to surgery and radiotherapy are poor. Recently, we experienced a case of primary mediastinal immature teratoma with elevated serum [-HCG and [-fetoprotein in 18 years old man. The well-encapsulated mass, weighing 4.5 kg, was completely resected and then adjuvant combination chemotherapy was tried with Vincristine, Bleomycin, and Cisplatin. Radical excision of tumor and adjuvant chemotherapy would appear to produce better result than have been reported in other cases. The postoperative course was uneventful and the tumor markers were returned to normal range.