Threshold image intensity for reconstructing patient-specific vascular models is generally determined subjectively. We assessed the effects of threshold image intensity differences on computational fluid dynamics (CFD) using a simple method of threshold determination. This study included 11 consecutive patients with internal carotid artery aneurysms collected retrospectively between April 2009 and March 2010. In 3-dimensional rotational angiography image data, we set a line probe across the coronal cross-section of the parent internal carotid artery, and calculated a profile curve of the image intensity along this line. We employed the threshold coefficient (Cthre) value in this profile curve, in order to determine the threshold image intensity objectively. We assessed the effects of Cthre value differences on vascular model configuration and the wall shear stress (WSS) distribution of the aneurysm. The threshold image intensity increased as the Cthre value increased. The frequency of manual editing increased as the Cthre value decreased, while disconnection of the posterior communicating artery occurred more frequently as the Cthre value increased. The volume of the vascular model decreased and WSS increased according to the Cthre value increase. The pattern of WSS distribution changed remarkably in one case. Threshold image intensity differences can produce profound effects on CFD. Our results suggest the uniform setting of Cthre value is important for objective CFD.