Ductus arteriosus aneurysm is rare in adults and preoperative diagnosis has not been usually done. We report 2 cases of adult type ductus arteriosus aneurysm. In both cases, 3D computed tomographic scanning showed a saccular aneurysm originating from the distal aortic arch toward the left pulmonary artery, which had a notching in the orifice of the aortic side. They were successfully treated surgically though one was a ruptured aneurysm to the left pulmonary artery. In these cases, 3D-CT scan was of great value in the preoperative diagnosis of the ductus arteriosus aneurysm.