A 16-year-old female Poodle with multiple mammary masses was referred to the Veterinary Medical Teaching Hospital of Seoul National University. Radiographic finding indicated a round mass of soft tissue density at the ventral portion of the 4th-7th sternum, which was 7 cm in diameter and presented multiple mineralization within the mass. Cytologically, aspirate of the mammary mass was moderately cellular and revealed well-differentiated osteoblasts with moderately basophilic cytoplasm, various sized and eccentrically placed nuclei and distinct nucleoli. Osteoblasts interspersed with osteoid-suspected pink-staining intercellular matrix. Some cells displayed malignant features such as macronuclei, macronucleoli, binucleation, abnormal mitotic figures, anisocytosis and anisokaryosis. On which the mass was diagnosed as osteosarcoma. Histopathologic examinations of the mass were compatible with a diagnosis of osteosarcoma. The patient was treated with surgery alone. The patient died 45 days later from surgery.