Coronary artery aneurysm is a rare entity defined as expansion more than 1.5 times of normal coronary artery in diameter. A 77-year-old male was admitted to our hospital with a diagnosis of acute coronary syndrome and a giant right coronary artery aneurysm. Coronary angiography showed occlusion of the right coronary artery and significant stenosis of left anterior descending artery and left circumflex artery. Enhanced computed tomography( CT) showed a right coronary artery aneurysm with a diameter of 38 mm. Urgent coronary artery bypass grafting and coronary artery ligation proximal and distal to the aneurysm were performed. His postoperative course was uneventful.