Seventy five per cent of the osteosarcomas originate in the long bones. The skull bones are rarely involved and in our present knowledge there was no report of osteosarcoma arising from frontal bones in Iran. An 11 years' old male German shepherd dog was referred to the Small Animal Hospital, faculty of Veterinary medicine, because of a mass on the head of two months duration. The lesion was a hard nodule on the left Frontal bone, measuring 10 cm in diameter. There was mild tenderness on palpation and no warmer than body temperature. The owner gave no history of head trauma or fractures. A CBC, biochemistry work-up, and urinalysis showed no abnormalities. Melena was positive in the stool because of passage of blood through the nasal cavity and pharynx. The neurological examinations were normal, excluding brain invasion. Lat. oblique view of the skull showed cortical destruction of the left frontal bone with amorphous extra cortical new bone formation extending into the soft tissue neoplastic mass. The lesion seems to be more osteoclastic. Right frontal bone is intact. Lat. oblique radiography revealed more aggressive changes in left frontal bone which shows rapid rate of changes of this malignant lesion. Microscopic examinations revealed highly cellular neoplastic tumor, compose of fairly uniform stellate vacuolar or slightly eosinophilic cytoplasm. The nuclei are irregular in shape and vesicular with one or two prominent nucleoli. The Tumor cells tend to produce osteoid tissue. Numbers of giant osteoclastic cells around osteoid formation are evident. Occasionally spicules of bone formation also present. After radiographic and pathologic confirmation of osteosarcoma and because of owner request dog was euthanized.