Coronary artery aneurysms (CAAs) are rare, and aneurysms of the left main coronary artery are extremely rare. Atherosclerosis is the main cause of CAAs. An 87-year-old man with atypical chest pain of 1 month's duration presented to a hospital. Physical examinations of the patient's cardiopulmonary system and vital signs were unremarkable. Electrocardiography showed a right bundle branch block and an atypical ST-segment elevation in the inferior leads. Coronary angiography demonstrated a huge aneurysm in the left main without any clots. Additionally, there were no remarkable findings in cardiac monitoring and spiral chest computed tomography scanning. Ultimately, after he received nitroglycerin, β-blockers, Ca++ channel blockers, Aspirin (ASA), and clopidogrel (Plavix), his pain was relieved.