: A potential interaction between the local hemodynamics and the artery wall response has been suggested for vascular graft failure by intimal hyperplasia (IH).Among the various hemodynamic factors, wall shear stress has been implicated as the primary factor responsible for the development of IH. In order to explore the role of hemodynamics in the formation of IH in complete bypass graft, code ADINA is used.The fluid flow through a stenosed artery in a complete bypass can influence the outcome of bypass surgery. To help improve our understanding of this , the steady and unsteady Navier-Stokes flows are computed in a 3-D idealized arterial bypass system with partially occluded host artery. At first steady flow in a complete bypass tube was simulated. The study was to consider a complete flow field, which included both the bypass and the host tubes. The changes of the hemodynamics were investigated with three parameters:the inlet Re,anastomotic angle and the position of the occlusion . The baseline flow field was set up with Re=200, ? =45 and the centered position of occlusion. The parametric study was then conducted on combination of Re=100, 200, 400, ?=45 , ?=60 and three occlusion positions: left, center and right. The separation points were on the opposite walls to the junctions. Recirculation zones were also found near the toe and in the proximal outer wall of the bypass tube. In some cases, pairing vortices could be seen in the host tube upstream of the occlusion. The shear rate distribution associated with the flow fields was presented. The flow pattern obtained was satisfying to those observed experimentally by other investigators. The difference of the flow fields between a complete bypass and simple anastomosis was discussed.Four geometric models, including symmetric stenoses 0,56,75,100% in the host artery, and a major aspect of the bypass system, namely, the effect of area reduction are considered. By analyzing the flow characteristics in these configurations, it is found that (1) substantial area reduction leads to flow recirculation in both upstream and downstream of the stenosis and in the host artery near the toe, while diminishes the recirculation zone in the bypass graft near the bifurcation junction and (2) the curvature of the bypass graft can modify the fluid flow structure in the entire bypass system. Finally, pulsatile waveforms were introduced to study the hemodynamics in a bypass anastomosis under unsteady flow condition.Time-dependent and steady state results were compared to gain an insight towards the feasibility of using steady condition as a representative of the averaged flow behavior within arteries. Keywords :bypass,intimalhyperplasia, anastomotic angle, wall shear stress, stenosis, pulsatile flow.