Ovarian smooth muscle tumors are a very rare type of ovarian tumor. In this paper, we report the case of a 62-year-old woman who had a huge smooth muscle tumor of the right ovary. The values of all the serum tumor markers were within normal limit. The tumor measured 25 cm in diameter and weighed 6,200 g. Histological examination revealed that coagulative cellular atypia was moderate to severe, necrosis was not present and mitotic index was low. According to the criteria for the evaluation of the uterine smooth muscle tumors, this huge tumor was diagnosed as atypical leiomyoma. However, we finally made a diagnosis of this tumor as a smooth muscle tumor of uncertain malignant potential (STUMP) because of its huge size. Further information is required regarding the characteristics of ovarian smooth muscle tumor and the propriety to introduce uterine tumor histological criteria to ovarian tumors.