The material of this study is a 3.5 years old male Rottweiler dog presented to Istanbul University Veterinary Faculty Surgery Clinics with a complaint of swelling in the right shoulder region. A diagnosis of hematoma was made and treatment was performed, but swelling started to grow gradually and general well-being deteriorated. When the drain was controlled, drainage of fluid containing blood was observed. Anemia and leucocytosis was determined on examination of the blood. The patient was operated. The mass was observed to expand in the thoracal cavity and it was found that organs in that regions were disfunctioned. So the patient was considered to be inoperable and euthanasia was performed. Specimens taken were examined histopathologically by using streptavidin-biotin immunperoxidase (IMP) method and a diagnosis of hemangiosarcoma of soft-tissue origin was made.