Struma ovarii is a monodermal teratoma composed totally or in overwhelming proportion of thyroid tissue. It is uncommon and accounts for 1% to 3% of benign teratomas of ovary. Grossly, they are encapsulated neoplasms, several centimeters in diameter and have a red, shiny, and meaty surface. Microscopically, the tumor is composed of mature thyroid tissue consisting of various sizes, lined by a single layer of columnar or flattened epithelium. The clinical behavior is benign, and simple excision is adequate treatment. We had experienced two cases of struma ovarii arising in the left ovaries. So we report above cases with brief review of literatures.