In order to study effect of extracellular Ca²⁺ and ionophore A23187 on erythrocyte deformation, the new ektacytometry is used. After erythrocytes were treated with different concentrations of intercellular Ca²⁺ and ionophore A23187 respectively, the orientation index (DI)_or and small deformation index (DI)_d of the erythrocytes were measured. It is suggested that the effect of ionophore A23187 on the rheology of erythrocytes is more notable than that of intercellular Ca²⁺. Furthermore, the (DI)_or,max and (DI)_d,max decrease as the concentration of intercellular Ca²⁺ or ionophore A23187 is increased. The deformability of erythrocytes was obviously reduced after treated with ionophore A23187.