The circle of Willis (COW) is an important intracranocervical collateral circulation system. We hypothesized that the integrity of COW may affect the characteristics of carotid plaques by influencing carotid hemodynamics. This study investigated the relationship between incompleteness of COW and the compositional features of atherosclerotic plaques in carotid arteries. We found that the incompleteness of circle of Willis is associated with vulnerability of carotid artery atherosclerotic plaques. Our findings suggest that integrity of circle of Willis may play a role in occurence of high risk plaque features, particularly intraplaque hemorrhage and fibrous cap rupture.