Primary leiomyosarcoma of the pulmonary artery is a rare malignancy arising from the multipotential mesenchymal cell of the intima of the pulmonary artery. Due to its rarity and nonspecific clinical symptoms, the correct diagnosis and proper management are often delayed. Furthermore, it is frequently misdiagnosed as pulmonary embolism, mediastinal mass, pulmonary stenosis and lung cancer. Therefore, it is important to consider primary leiomyosarcoma of the pulmonary artery a possibility when a persistent filling defect is present in the pulmonary artery and there is no response to optimal anticoagulation treatment. Radiologic findings such as a unilateral mass continuously filling the pulmonary artery, inhomogenous enhancement, vascular distension, extravascular invasion into adjacent structure or uptake in the area of tumor on the FDG-PET can be helpful when differentiating pulmonary artery sarcoma (PAS) from chronic thromboembolism.