In the immature teratoma with increased serum alpha-fetoprotein (AFP), a complete pathologic examination is especially required because coexistence with other germ cell tumors is associated with a poor prognosis. If the case is proved to be a pure immature teratoma in spite of a thorough examination, the source of AFP should be found as an AFP-producing pure immature teratoma is not associated with a poor prognosis. In this case of a grade III-immature teratoma in an ovary of a 12-year old girl, serum AFP was increased. On pathologic examination, there was no evidence of a yolk sac tumor or embryonal carcinoma. On an AFP immunohistochemical stain, immature liver tissue, digestive and respiratory epitheliums were positive.