A 41-year-old woman complained of dyspnea due to pneumonia and congestive heart failure and was diagnosed as severe mitral stenosis and insufficiency. She was previously diagnosed as antiphospholipid syndrome (APS) because of previous pregnancy morbidity and cerebral infarction. Mitral valve replacement was performed with postoperative strict anticoagulant therapy and postoperative course was uneventful. Cardiac surgery in APS patients has been reported with high morbidity and mortality because of perioperative fatal thromboembolism. It is very important to initiate anticoagulant therapy with heparin immediately after the operation to prevent thrombosis.