Cerebrovascular diseases are one of the three major mortalities in Japan, such as the rupture of cerebral aneurysm and cerebral infarction caused by carotid stenosis. The growth mechanism of the cerebral aneurysm and carotid stenosis has not been clearly understood. In this research, we are introducing a numerical simulation tool; Computational Fluid Dynamics (CFD) technique, to simulate and predict the hemodynamics of blood passing through the cerebral aneurysms and stenosed carotid arteries. The results of a ruptured and an unruptured cerebral aneurysm were compared. Energy losses were calculated in ruptured and unruptured cerebral aneurysms, the results were 167 Pa and 6.3 Pa respectively. The results also indicated that the blood flows took longer residence inside of bleb of the ruptured aneurysm. The maximum wall shear stress was observed at 70% stenosis from the simulation results of stenosed carotid bifurcation. The result qualitatively agrees with classical treatments in carotid bifurcation therapy.