A Case of main pulmonary artery aneurysm in a 9-year-old boy with patent ductus arteriosus is presented. In this case presented with a huge mass density on the chest X-ray, radionuclide cardiac angiography showed a vascular lesion, which was confirmed as an aneurysm of the main pulmonary artery at roentgenologic angiogram. The aneurysm appeared following an episode of bacterial endocarditis and pulmonary hypertension. A successful aneurysmectomy with multiple ligation of ductus arteriosus was performed.