We numerically studied blood flows in a simulated branching pipe of coronary artery bifurcation, which is affected by stent implantation. We found that stent struts provide effects as guide vanes and blockages on the flow into circumflex branch. The former effect increases the flow rate and shear stress on the arteriosclerosis-prone site. The blockage effect may overwhelm the guide effect depending on a strut position against the inflow.