The patient was a 15-month-old girl with Down's syndrome. She had a heart murmur on the first day after birth. The echocardiogram revealed that she had the tetralogy of Fallot (TOF) and mitral insufficiency (MI). She was observed because she had no heart failure or cyanosis. However, she developed heart failure with progressive MI. Then, she was admitted to our medical center for surgical treatment. During the operation, it was confirmed that the primary septum was intact and a large ventricular septal defect was located at the inlet to outlet portion with anterior malalignment. Each leaflet of the atrioventricular valve were attached to the same level and the ventricular septum was scooped out. TOF with endocardial cushion defect (ECD) without primary septal defect was diagnosed based on the operative findings. Surgical repair was performed through the right atrium and pulmonary artery. She was discharged 17 days after operation without any complications. This was a very rare combination of TOF with ECD without a primary septal defect. We discussed this rare condition with a review of the literature.