Background: Struma ovarii (literally: goitre of the ovary) is a rare form of monodermal teratoma that comprises either entirely or predominantly thyroid tissue (>50%). This also includes cases of mature teratoma with less than 50% thyroid tissue but harboring thyroid-associated malignancy. This study was to investigate the biological feature and immunohistochemical features of Struma ovarii. Patients and methods: This retrospective study enrolled 24 patients with Struma ovarii from 1986 to 2016, including 1 case of papillary thyroid carcinoma and 23 cases of benign struma ovarii. Results: The median age was 49.2 (ranged 24-79). Concurrent with primary ovarian lesions, 2 cases had ipsilateral mucinous cystadenoma, 8 cases had ipsilateral cystic teratomas and one case had ipsilateral mature cyst teratoma, whereas one case had contralateral Brenner tumor. Ascitic fluids were observed in four cases and hydrothorax fluids were observed in two cases. Nine cases were found by chance during a routine physical examination. The most common clinical manifestations were lower abdominal discomfort and palpable mass (10/24), menorrhagia (2/24), acute hypogastralgia (congenital ovarian cysts torsion, 3/24), cough and polypnea (2/24). Elevated CA125 was found in four cases. The tumors of all the cases had totally intact encapsulation. Tumor size ranged from 5-15.5 cm (average, 10.3) in greatest dimension. Twenty were multilocular, other four were unilocular. The cystic masses were typically filled with light yellow-brown or amber gelationous tissue and clear green-yellow fluid. Microscopically, the tumors consisted of thyroid follicular structure that were lined by flatcuboidal cells and filled with eosinophilic colloid-like material. In one case, the epithelium cells exhibited malignant transformation, which was classified as a follicular variant of papillary carcinoma. Furthermore, endometrial polyps were found in two cases, one of them was post-menopause. Two cases were initially misdiagnosed as other ovarian tumors during intraoperative frozen section. Immunohistochemical stains for thyroglobulin and thyroid peroxidase were positive. Thirteen patients were cured without relapse and one patient was lost to follow-up. Conclusions: Struma ovarii is rare tumor and highly specialized form of monodermal ovarian teratoma, with a peak incidence in the fifth decade of life. Struma ovarii may manifest variant pathologic features presenting in the thyroid gland. They should be diagnosed on the same histological basis as thyroid carcinoma. Immunohistochemical staining for thyroglobulin and thyroid peroxidase contributed to the diagnosis of this disease. Principle of clinical treatment of malignant struma ovarii is commonly derived from the experience with thyroid carcinoma. Postoperative adjuvant radiation therapy and chemotherapy may be recommended to perform. The prognosis is generally favorable.