A computer simulation using particle method was carried out to investigate effects of deformabilities of red blood cells (RBCs) on blood flow in microcirculation. The motion of deformable RBCs was determined by the spring model on the basis of the minimum energy principle. The motion of the plasma fluid particles was analyzed by the moving-particle semi-implicit (MPS) method assuming incompressible viscous flow. The two-dimensional simulation of blood flow between parallel plates demonstrated that RBCs moved downstream and were deformed into a parachute shape as observed in experiments. Comparison between the results for single RBC and multiple RBCs clarified that the deformation of RBCs was suppressed by their neighboring RBCs. The higher the spring constants of the RBC model, the less deformed the RBCs were, causing large blood flow resistance. These results indicate that deformability of each RBC cooperates with mechanical interaction between RBCs in determining the rheological properties of blood on blood cellular scale.