Ovarian mucinous tumors mixed with Brenner tumors have rarely been reported. The coexistence of such epithelial tumors present histopathologic diagnostic difficulties. Here we report a 57-year-old postmenopausal woman who had experienced an abdominal distention and pain over a period of eight months. A physical examination revealed a grossly distended abdomen that reached the xiphoid process. A firm and mobile abdomino-pelvic mass with a smooth surface and a regular border was identified. Laboratory investigations showed a hemoglobin level of 13.54 g/dl and a serum cancer antigen 125 (CA125) of 97.3 U/mL. Trans abdominal ultrasonography revealed a massive complex mass originating from the left adnexa. A laparotomy was performed and a 10 kg left adnexal mass was removed intact. Histopathological analysis revealed mixed benign mucinous cystadenoma with a Brenner tumor of the left ovary. After surgery the patient showed marked clinical improvement, resumed her regular daily activities in three months and no recurrence has occurred during her long follow up. As the coexistence of these mixed tumors is not uncommon, a thorough pathologic evaluation is necessary and health professionals should be aware of the mixed occurrence of epithelial ovarian tumors.