The paper aims to investigate the relationship between aneurysm's geometry and hemodynamics in order to predict rupture and creation of cerebral aneurysms. The authors have been developing a numerical simulation system, which consists of medical image-based geometric modeling, grid generation, finite element fluid simulation, and scientific visualization. The simulation was conducted for 20 cases of the middle cerebral artery (MCA) aneurysms (17 unruptured aneurysms and 3 ruptured aneurysms). The wall shear stress distributions and flow patterns are compared with respect to the aspect ratio. The results show that as the aspect ratio becomes larger, the wall shear stress distribution in the aneurysm becomes smaller.