Congenital left atrial appendage aneurysm is a very rare congenital cardiac abnormality. That is postulated to arise from a developmental weakness in the atrial wall in utero. Clinically, patients are often asymptomatic and are diagnosed incidentally, but supraventricular arrhythmias and systemic thromboembolism have also been reported in some cases. Surgical resection at the time of diagnosis is recommended because of the propensity for thromboembolic complications. A 13-month-old female, who was suspected preoperatively as having partial absence of pericardium with left atrial herniation through the defect, underwent surgical resection of the left atrial appendage aneurysm. Exposure through a median sternotomy showed an intact pericardium. The postoperative course was uneventful.