We successfully performed resection of a large asymptomatic left atrial appendage aneurysm( LAAA) under non-cardiac arrest. Although the patient was asymptomatic, chest X-ray showed projection of the third left aortic arch, and computed tomography angiography showed a huge left atrial appendage aneurysm. Therefore, we decided to perform LAAA resection via a partial median sternotomy. Aneurysmectomy was performed by placing the patient on pump beating heart. The size of the resected aneurysm was approximately 80 mm in diameter. Because the base of the LAAA was constricted and the tissue was not fragile, resection was performed using an automatic excision anastomosis device( Echelon Flex). The patient's postoperative course was uneventful. Left atrial appendage aneurysm is believed to be congenital;however, it is often detected in adulthood. Long-term follow-up is necessary to ensure maintenance of optimal cardiac function.