A rapidly growing mandibular tumor occurred in a 17 month old female infant. Tumor outgrowth showing a periosteal reaction was radiographically seen on the lower surface (base) of the mandible. Under the biopsy diagnosis of osteosarcoma, high-dose chemotherapy with methotrexate was performed, resulting in little effect. The right hemimandibulectomy specimen disclosed intraosseous infiltrative growth of pleomorphic adenoma of salivary gland type, associated with chondroid stroma and reactive bone formation. The highly proliferative small-sized cells retained immunohistochemical features of myoepithelial cells, with positive reactivity of cytokeratin, vimentin, S-100 protein, alpha-smooth muscle actin, epithelial membrane antigen, CA15-3, type IV collagen, laminin and p53 protein. No heterotopia of the salivary gland was identified within the bone tissue. The tumor recurred 2 months later. Due to uncontrollable local growth, the patient died 8 months after operation. At autopsy, reactive ossification was closely associated with malignant myoepithelial proliferation. No distant metastasis was noted. This osteosarcoma-like tumor can be regarded as myoepithelial carcinoma in pleomorphic adenoma, originating from intramandibular heterotopic salivary gland tissue.