Mural endocarditis causing myocardial abscess without valvular involvement is very rare. We report an unusual case of left atrial auricular abscess which was successfully treated by surgical resection, treatment with antibiotics, and mediastinal irrigation. A 9-yr-old female patient with previous history of urinary tract infection was admitted because of persistent fever. Echocardiography and magnetic resonance imaging revealed massive pericardial effusion and a mass lesion at the left upper cardiac border. Pericardiocentesis isolated Staphylococcus aureus on culture. The patient underwent mass removal under cardiopulmonary bypass. The mass was located in the left atrial auricle with fibropurulent abscess formation inside. Postoperative mediastinal irrigation was performed using povidone iodine solution. Pathological examination of the mass showed organized thrombi with chronic fibrosing mural endocarditis.