Aneurysm of both the pulmonary trunk and the ascending aorta concomitant with bilateral bicuspid valves is very rare. The reason for the formation of aneurysm with bicuspid semilunar valve is still inconclusive. Surgical repair was performed successfully (ie, aortic valve replacement, graft replacement for the ascending aorta, and plication of the pulmonary artery). Pathology of the pulmonary artery wall did not demonstrate cystic medial necrosis. The hemodynamic turbulence by the bilateral bicuspid valve may cause the formation of aneurysms even at low pressure. This case demonstrates an explanation for aneurysm with the bicuspid valve.