This research compared the intraoperative appearance, computational fluid dynamic (CFD) analysis, and scanning electron microscope (SEM) observation of endothelial cells (EC) of seven human cerebral aneurysms in an effort to find the relationship between hemodynamic patterns and wallthinning of aneurysms. Aneurysm walls were categorized into blister-like appearance as thinning, similar appearance to the parental artery or white thickened part as thickening, and areas similar to arteriosclerosis as calcifying region. Seven aneurysms were dissected after clipping operation with markings to confirm the orientation of specimens. Specimens were rinsed in PBS, immersed into 2% glutaraldehyde, 1% osmium solution, graded series of ethanol and tertiary-butyl alcohol, and freeze-dried to observe under SEM. For CFD analysis, 3 dimensional vascular models were constructed and meshed using tetra and prism mesh. The inlet boundary condition used an average flow rate of 350 ml/min. The outlet boundary conditions were set to flow rates based on literature data and for regions with unknown flow rates were set to static pressure. Vascular models were analyzed using CFX 13.0 (ANSYS). Aneurysms contained four thinning, six thickening, and four calcifying regions. SEM analysis showed variations of endothelial cell patterns over aneurysms. Missing EC was only found at thinning regions and all thickening and calcifying regions confirmed damaged or normal EC. CFD analysis showed impinging flow at the thinning region. Impingement was defined by the direction of wall shear stress (WSS) vector and rise of pressure at wall. The impinging flow was only found at the thinning region. The WSS magnitude did not confirm any relationship between wall characteristics or EC patterns. These results suggested that the impinging flow may cause wall thinning of aneurysm.