We report a monochorionic diamniotic twin pregnancy with prenatal diagnosis of aortopulmonary septal defect combined with type B interrupted aortic arch in one of the fetuses. The mother was referred for fetal echocardiography at 24 weeks' gestation due to suspected congenital heart disease. Prenatal echocardiography revealed a defect of 2.8 mm between the main pulmonary artery and the ascending aorta. The course of the ascending aorta was straight to the neck and head, and there was no continuity of the aortic arch after the origin of innominate and left common carotid arteries. Thus, aortopulmonary septal defect with type B interrupted aortic arch was suspected. Postnatal echocardiography confirmed the diagnosis, and surgical repair was performed on the 10th day after birth. The combination of aortopulmonary septal defect with type B interrupted aortic arch is a very rare condition that can be diagnosed by fetal echocardiographic examination in the second trimester of gestation. Prenatal diagnosis is important for the prognosis, since early surgical intervention is needed to prevent development of severe heart failure in the neonate.