A case of Juvenile Granulosa Cell Tumor (JGCT) of the ovary with unusual pleomorphic histologic and malignant biologic behaviour is described. The tumor occurred in a 10-year-old girl and was associated with clinical features of isosexual pseudoprecosity and a marked elevation of serum estradiol. The mass manifested initially in the right ovary and subsequently involved the contralateral ovary. A multi-organ metastasis developed during a 6-month-interval despite chemotherapy. She received two operations at 6-month interval, and tissues were obtained from the tumor mass. A marked histologic difference was observed between these two samples. The second biopsy showed profound cellular pleomorphism with numerous multinucleated tumor giant cell formation and hyaline bodies. The differential diagnosis from germ cell tumor and the possible factors for the pleomorphism are discussed.