About 20~30% of benign or malignant tumors of ovarian origin arise from embryonic cells, and only 3% represent malignancy. But under age of 20, 70% of ovarian tumors arise from embryonic cells, and over 1/3 of them are malignant tumors. Over all the ovarian tumors arising from embryonic cells, immature teratoma is germ cell tumor, components include immature tissues and cells derived from ectoderm, mesoderm, and endomermal origins. Most of the immature tissues are from neuroectodermal origins. The immature teratoma of the ovary is a rare tumor, representing less than 1% of all ovarian neoplasm. These tumors typically present in young age woman (mean age 10~20 years) with pelvic and abdominal pain. Nowadays newly developed combination chemotherapeutic agents such as bleomycin, etoposide, cisplatin give us great survival and disease free prognosis than before. We have experienced two cases of immature teratoma so we report them with a brief review of concerned literatures.