We report a case of idiopathic left atrial appendage (LAA) ostial stenosis found incidentally by transesophageal echocardiography (TEE) prior to radiofrequency catheter ablation for atrial fibrillation. A 65-year-old male had a history of paroxysmal atrial fibrillation (PAF), which had been treated for two years. He was referred to our department for the treatment of drug-refractory atrial fibrillation. TEE performed to search for an atrial thrombus showed a narrowed LAA orifice with a diameter of 3 mm. Color Doppler showed a turbulent flow across the LAA orifice. The peak velocity of the LAA orifice determined by continuous wave Doppler evaluation was 1.2 m/sec. Mild spontaneous echo contrast was observed in the left atrium, but no thrombus was found in the LAA and left atrium. After admission, pulmonary vein isolation with radiofrequency catheter ablation was performed. He had no relapse of atrial fibrillation after ablation therapy. With the exception of incomplete surgical ligation of the LAA, idiopathic LAA ostial stenosis is a rare entity. The etiology and clinical implications of this entity remain unclear. Further investigation will be required to confirm the clinical consequences of LAA ostial stenosis.