Objectives: Factor(s) affecting the sac size of an abdominal aortic aneurysm (AAA) after endovascular aneurysm repair (EVAR) remain unclear. We compared the diameter of the aneurysm sac at one year after surgery with the preoperative diameter using CT images.Methods: Patients who underwent EVAR at Juntendo University Hospital were involved. According to the size change in treated lesions of the aorta, patients were categorized into the following 3 groups: shrink (<5 mm of reduction), enlarge (>5 mm of expansion), and no change (size change within 5 mm). The patients’ background, laboratory data, devices used, medications, anatomical characteristics, and presence/absence of postoperative endoleaks were examined.Results: Of the 68 consecutive patients, 23 were excluded. Seventeen patients were classified into the shrink group, 28 patients into the no change group, and no patients into the enlarge group. Patients with higher thrombotic area rate on the preoperative AAA tended to present AAA sac shrinkage (p = 0.05). No other variables affected the size change in this study. In addition, the existence of an endoleak suggested the interference of sac shrinkage.Conclusions: The higher AAA thrombotic area rate tended to associate with AAA sac shrinkage.