Mycotic pseudoaneurysm of the aorta is a rare disease in children. We report our experience with a postoperative mycotic pseudoaneurysm of the ascending aorta secondary to mediastinitis after a modified Fontan procedure. The pseudoaneurysm was successfully repaired using an autologous pericardial patch through a right thoracotomy under total circulatory arrest. During 6 months of postoperative follow-up, there were no recurrences of pseudoaneurysm formation.