Malignant ovarian tumors in children are very rare, and consist of about 1% of all childhood malignant tumors. The purpose of this study is to examine the clinical characteristics, treatment, and prognosis for children with malignant ovarian tumors. We retrospectively reviewed the medical records of children under 15 years of age with malignant ovarian tumors who had been treated surgically at Asan Medical Center between 1989 and March 2009. There were 32 patients, ranged in age at surgery from 2 to 15 years (mean; 10.4 years). The median follow-up period was 64.7 months (from 1 month to 188 months). Pathologic diagnosis were; immature teratoma (n=10), mixed germ cell tumor (n=10), and dysgerminoma (n=6). Tumor stage was classified by the staging system of the International Federation of Gynecology and Obstetrics (FIGO). The number of patients in stage I, II, III, and IV were 24 (75%), 2 (6.2%), 4 (12.5%), and 2 (6.1%), respectively. The tumor recurred in 4 patients. Seven patients of group 1 did not receive postoperative adjuvant chemotherapy, and in three of them, the tumor recurred. Twenty-five patients (group 2) underwent postoperative adjuvant chemotherapy, and there was only one recurrence. One patient who did not receive postoperative adjuvant chemotherapy and expired 10 months after operation because of tumor recurrence and distant metastasis. The overall 5-year event free survival (EFS) was 84.2%: group 1 in 44.4%, and group 2 in 95.7%. Tumor recurrence was related to the postoperative adjuvant chemotherapy (p=0.004). In conclusion, proper surgical procedures with relevant postoperative adjuvant chemotherapy might improve clinical results in children with malignant ovarian tumors.