This chapter explains important characteristics of blood flows in microcirculations and outlines the modeling of blood cells and the results of numerical simulations for blood flows containing blood cells. Biological cells have lipid bilayer membrane. Human blood is composed of biological cells, proteins and plasma. Although behavior of red blood cells (RBC) is mainly discussed so far, not only RBCs but other blood cells, such as white blood cells, platelets, and drag delivery agents regarded as artificial blood cells, need to be treated in the same manner. Compared with spherical blood cells such as neutrophil, RBCs are highly deformable and can move smoothly into small vessels. Actual living cells including blood cells have not only lipid bilayer membrane but a protein network structure called cytoskeleton underneath the lipid bilayer is cytoskeleton structure gives the strength of membrane under the strong shear flow. Due to the presence of cytoskeleton structure, the membrane of blood cells no longer behaves as fluid membrane.