Cryoablation was performed simultaneously with mitral valve plasty for a 65-year-old man with atrial fibrillation and mitral insufficiency. The sites of cryoablation were determined during atrial fibrillation using a computer-atrial-mapping system when the operation was performed. The site of repetitive activation was found at the area between the left atrial appendage and the superior left pulmonary vein. That area and the surrounding area were cryoablated five times (-60°C, 5min). After operation, normal sinus rhythm returned without the continuous use of any antiarrythemic drugs. Furthermore, the patient who underwent mitral valve plasty, does not need any anticoagulant drugs. This procedure was very effective and had little operative risk in this case. The patient is doing well with normal sinus rhythm 6 months after the operation.