A case of an ovarian mucinous cystadenocarcinoma with mural nodules of anaplastic carcinoma is reported. The patient was a 46-year-old female with a right ovarian cystic mass and underwent a total abdominal hysterectomy with bilateral salpingo-oophorectomy. The patient died of the disease in five months. Macroscopically, the right ovarian cyst wall contained numerous well-demarcated mural nodules. Histologically, the cysts were mucinous cystadenocarcinoma, and the nodules were composed of sarcoma-like spindle and polygonal cells with atypia and numerous mitoses. Spindle cells in the mural nodule showed epithelial characteristics by electron microscopy and immunohistochemistry. This case supports an epithelial origin of the mural nodule of anaplastic carcinoma in ovarian mucinous tumors.