Mantle sign and anechoic crescent (AC) sign are two representative ultrasound findings of aortic aneurysm. Mantle sign is a hypoechoic wall thickening of the anterior or anterolateral side of an aortic aneurysm, and suggests that the aortic aneurysm is inflammatory. AC sign is a crescent-shaped anechoic area between abdominal aortic aneurysmal wall and mural thrombus, and is sometimes misdiagnosed as aortic dissection. These signs are clinically useful; however, differential diagnosis from similar findings is often challenging. Close observation of ultrasound images is necessary for the correct diagnosis along with medical history, blood analysis, and other imaging modalities. We described important points for differential diagnosis using these signs, and the origin and clinical significance of these signs.