Endodermal sinus tumor[EST] of the mediastinum is a rare germ cell neoplasm. It usually arises from the ovaries and testes but also arises from multiple extragonadal site including the mediastinum. Characteristically, alpha feto protein level is high and used for monitoring the clinical course. EST of the mediastinum is poor prognosis because of its direct invasion. The patient was 18 month old female with chief complaints of cough and fever. In the chest X-ray and CT, large encapsulated, 7x6cm sized, mass of anterior med-iastinum was found, and we could excise it completely because it was well encapsulated and not invaded but only adhered to aortic arch, pericardium and left upper lung. And confirmed it as EST by histopathology. Pre-operative alpha feto protein[AFP] level as 41,748ng/ml and decreased to 2, 663ng /ml at 14th postoperative day, 644ng /ml at 31th postoperative day. From 17th post-operative day, chemotherapy was started and keep going now.