The case is about a 42-year-old woman, who complained of a painful mass of her right breast. The mammography demonstrated a high-density mass lesion of size 2.7 x 2.2 x 1.7 cm, which was presented not well-circumscribed, irregular borders (Figure 1). The mass showed heterogeneous calcification. For making pathological diagnosis of the mass, core needle biopsy was taken. The histopathology of the tumor was intriguing, because there were 2 different components in the tumor. One of them was typical invasive ductal carcinoma (IDC) (Figure 2), the other one was presenting as osteosarcoma. Lots of immature trabeculae could be seen, and atypia osteocytes among them (Figure 3). The results of immunohistochemistry showed different presentations between them. The IDC parts of the tumor was positive for ER, PR, and CK8/18, but the osteosarcoma was negative for ER, PR, and Her-2. The component of osteosarcoma expressed SATB-2 (Figure 4). The patient underwent the radical operation of the lump. The gross appearance of the tumor revealed a solid mass with hemorrhagic (Figure 5). The mass was too firm to make the sections, so it was been decalcified. Routine microscopic examination revealed an invasive biphasic tumor comprising malignant epithelial and osseous tissue. It was the same as the histopathology of the core needle biopsy, as well as the results of immunohistochemistry. The final diagnosis was given, that was breast mixed metaplastic carcinoma, which was display a mixture of osteosarcoma (65%), with invasive ductal carcinoma (35%).