Primary mediastinal endodermal sinus tumor (or yolk sac tumor) is very rare and malignant tumor and has been reported by only several authors. Mediastinal endodermal sinus tumor located in anterior section is histologically compatible with endodermal sinus tumor located in genital tract. This tumor responds relatively well to chemotherapy and the pure form has the worst prognosis. We experienced 2 cases of primary mediastinal endodermal sinus tumor which affected 19yrs and 18yrs old males. The first patient has been treated with four cycles of BEP combination chemotherapy (cisplatinum, vp-16, Bleomycin) and followed by other treatment for brain metastasis, The 2nd patient received the 1st cycle of BEP chemtherapy and followed up.