Primary pulmonary artery angiosarcoma is very rare. We present a case of primary pulmonary artery angiosarcoma that was diagnosed as pulmonary hypertension and subacute worsening right-sided heart failure in a 59-year-old woman. Constrast-enhanced helical CT showed large mural defects in both main pulmonary arteries, clearly delineated by contrast medium. The ¹⁸F-fluorodeoxy glucose positron emission tomography findings of pulmonary artery angiosarcoma were differentiated from those of pulmonary thromboembolism. This tumor was diagnosed at autopsy. Histopathologically, the excised tissue revealed a highly cellular growth of atypical spindle cells with immunohistological findings: positivity for factor VIII-related antigen, CD31, CD34, and UEA-1. Although the incidence of primary pulmonary artery angiosarcoma is very low, our case demonstrates that this disease entity should be included in the differential diagnosis of pulmonary thromboembolism. Patients with early FDG-PET can have curative potential with aggressive surgical intervention.