We report here a rare case of osteosarcoma with fibroadenoma of the breast. A 56-year-old woman was admitted to the hospital because of a rapidly growing tumor in her left breast. On physical examination, a hard multilobular tumor measuring 6×6cm with well-circumscribed margin was detected in the inner half of the left breast. Mammography revealed both an irregular heterogeneous tumor with an acoustic shadow and an anechoic area, and a regular homogeneous tumor which was adjacent to the irregular tumor. Fine needle aspiration cytology suggested that the tumor was malignant. Intraoperative frozen section diagnosis was metaplastic carcinoma with cartilaginous differentiation typical of chondroma. Mastectomy with lymph node dissection was carried out. Pathologically, the tumors were diagnosed as osteosarcoma and fibroadenoma. As of one year after the surgery, there have been no recurrence and metastasis.