Intraoperative map-guided cryoablation for chronic atrial fibrillation and coronary artery bypass grafting were simultaneously performed in a 55-year-old man. Computerized atrial mapping revealed that regular and repetitive electrical discharges originated in the left atrial appendage. Without opening the left atrium, we successfully ablated atrial fibrillation with cryoablation on the epicardium of the left atrial appendage. Thus, intraoperative mapping should facilitate operations for atrial fibrillation.