In this paper, the rouleaux size dependence of viscosity in concentrated red blood cell suspension is derived analytically, by using reversible kinetic equation of coagulation, to reveal the effect of erythrocyte aggregation on the blood viscosity. In the limit of dilute concentration and low shear rate, the result does not reduce to Casson`s equation. In the end we argue Murata`s result on the shear rate dependence of viscosity. (author). 8 refs, 5 figs.