It is well known that red blood cells(RBCs)are suffered from chronic stresses in systemic circulation. The objective of this study is to clarify the effect of the aging of RBCs on rheological properties and hemolysis. Initially, RBCs age fractionation was performed by using a high-speed centrifugation(15[min]at1500[G]), then young and aged RBCs were suspended in plasma to adjust the hematocrit level of40[%]. After this pretreatment, the viscosity was measured by using a capillary type and a cone-plate type viscometers, respectively, and the hemolysis test was carried out by a seesaw type shaker. Results from these experiments showed that the viscosity of young RBCs measured by the capillary viscometer was increased by 10[%]as compared with that of aged RBCs.Under the condition of all shear zones, the viscosity was increased in case of using the cone-plate type viscometer. And the hemolytic level was increased twice as the aging. The data obtained in this study indicated that the ability of aggregation of RBCs was increased and the deformability of RBCs' membrane got lower with the aging. Furthermore, it was exhibited that the fragility of RBCs'membrane was increased with the aging.