A 13-year old male dog with right forelimb lameness, invisible swelling, anorexia and mild depression was referred to Veterinary leaching Hospital, College of Veterinary Medicine, Kyungpook National University. On forelimb radiographic views, periosteal and endosteal proliferation and cortical lysis and extremely aggressive and amorphous periosteal reactions were located along the metaphyseal region of the proximal humerus. Cytologic and histopathologic examinations after amputation showed anaplastic mesenchymal cells that produced osteoid changes, poorly differentiated osteoblast and osteoclast. And also, it included the neoplastic cells with large nucleoli, multiple nucleoli that differ in size and cytoplasm with several clear vacuoles. Based on the physical examination and laboratory, radiolographic, cytologic and histopathologic findings, this case was diagnosed as osteosarcoma of the proximal humerus.