Although skeletal tumor is predominantly a disease affecting elder large- and giant-breed dogs, its possibility in a small-sized, young dog can never be ruled out. A case of osteosarcoma involving the left proximal humerus in a 3.5 years old Beagle is described. With vicious systemic metastasis, the dog was euthanized with the survival time of 16 weeks after amputation of affected limb. In the management of canine osteosarcoma, early diagnosis and aggressive treatment including surgical excision and adjuvant chemotherapy are critical.