Marfan syndrome may be associated with various cardiovascular complications, but expanding abdominal aneurysm is a relatively rare complication. A 32 year old male patient was admitted to S.N.U.H. because of pulsating abdominal mass and back pain. The patient was diagnosed as a case of expanding abdominal aortic aneurysm and annuloectasia of aorta due to Marfan syndrome. Abdominal aneurysm was located from 3 cm below the origin of left renal artery to left common iliac artery. Abdominal aneurysm was operated successfuly. The size of abdominal aneurysm was about 2815cm. The pathological findings of surgically removed abdominal aortic wall showed loss of elastic fibers and fragmentation of fibers in media. A case of expanding abdominal aortic aneurysm due to Marfan's syndrome was reported with a review of literatures.