An extremely rare case of primary malignant hemangioendothelioma of the thoracic aorta in a 54-year-old woman was reported. The patient was diagnosed as having an aneurysm of the thoracic aorta, and was operated on accordingly. The defect of the aortic wall was closed with a patch of woven Dacron prosthesis. Malignant cells were found postoperatively in the wall of aneurysm. Further immunohistochemical studies of the specimen revealed positive for factor VIII-related antigen, thrombomodulin, and Ulex europaeus agglutinin-1. The data suggested hemangioendothelioma. Reoperation in the twelfth postoperative month was unsuccessful. She died of respiratory failure 13 month after the initial surgery. Immunohistochemical studies are required for the correct diagnosis of vascular tumors. An aggressive surgical intervention is necessary as the only means of treatment.