A rare case of ascending aortic aneurysm due to post stenotic dilatation associated with aortic valve stenosis in a 67-year-old man was treated surgically. Aortic valve stenosis with a systolic pressure gradient of 87mmHg was recognized. Thoracic aortogram and CTscan revealed dilatation of the ascending aorta which was 85mm in diameter. Because the ascending aorta was obviously dilated and its wall was thin at operation, the possibility of rupture was considered to be high. Aortic valve replacement using a 23mm SJM prosthetic valve was performed and graft replacement of the ascending aorta with a 28mm collagen-shield graft was carried out simultaneously. He was discharged in excellent condition on the 45th postoperative day. Pathological examination of the aortic wall revealed an apparently thin wall, but the three layer structure of the wall and elastic laminae were well preserved. Inflammation and atherosclerotic findings were not detected. It was concluded that post stenotic dilatation associated with aortic valve stenosis can develop aneurysm that eventually requires surgical treatment.