We simulated the flow behavior of a single red blood cell (RBC) passing through a two-dimensional stenosed microvessel. Blood flow was computed using the lattice Boltzmann method, and the fluid-membrane interaction between the flow field and deformable RBC was incorporated using the immersed boundary method. The vessel diameter and length are 20 and 50 μm, respectively. Two semicircular stenoses were arranged on the upper and lower walls. We investigated the influence of upstream RBC position of vessel width direction and inclination angle against the main flow on RBC deformation and flow resistance when the RBC flowed through the stenosis. The simulation results indicated two flow patterns depending on the upstream position and inclination angle. When the upstream position was near the vessel wall, the average flow resistance increased due to RBC migration to the vessel center. By contrast, when the upstream position was near the vessel center and the inclination angle against the main flow was 90°, the flow resistance increased due to the large RBC deformation. These results can assist in understanding the processes of circulatory diseases with stenosis.