In a 10-year old mixed dog, a $10{ imes}10{ imes}4$ cm mass was palpated around the left caudal thoracic mammary gland with abdomen expension. In hormonal assay, serum concentrate of progesterone was 0 pg/ml and serum concentrate of estradiol was 66.7 pg/ml. Radiological finding of the mammary gland showed higher density than that of soft tissue and ultrasonographic finding showed inhomogeneous in echogenicity. Light microscopic view of mammary gland showed neoplastic hyperplasia of epithelial and myoepithelial cells and showed some metaplastic lesion of these cells to cartilage or bone tissues with metastasis to the lung. This case was diagnosed as malignant mixed mammary tumor.