The teratomas seem to be the most common mediastinal tumor in Korea. However, the immature teratomas are the rarest type of these mediastinal teratomas. A case of immature teratoma of the mediastinum occurring in a newborn male is reported. He suffered from severe dyspnea since birth. Simple chest X-ray and chest sonogram revealed huge mass shadow in fight whole lung field. On 6th day after birth, the operation was done and the mass was completely excised. Biopsy showed elements of mature tissues derived from 3 germ cell layers including immature neuronal components. The baby is well without evidence of disease till now.