Congenital left atrial appendage aneurysm is an extremely rare heart malformation. Some researchers also call it left atrial appendage diverticulum. Here, we report a patient who was hospitalized due to paroxysmal palpitations that became worse for 2 months. Transthoracic echocardiography and contrast echocardiography suggest a left atrial appendage aneurysm with thrombosis. Magnetic resonance imaging showed long T2 and slightly short T1 signal at the top of the left atrial appendage aneurysm. Computed tomography examination showed that the left atrium was connected to the giant left atrial appendage. The patient refused surgery despite the high risk of thromboembolism. Key words: left atrial appendage aneurysm, congenital heart malformation, thrombus