The hemorheological performance of mammals and human will change with the time after the blood is withdraw from the subject. The whole blood apparent viscosity, erythrocyte deformability and aggregation of some mammals and human were measured. Our results showed that the hemorheological characteristics of mice, rats, guinea pigs, dogs and human began to change respectively after the threshold time of 0.5, 1, 3, 7, 12 h when the blood were stored in vitro. Moreover there exist a relationship: t=3.18W(0.32) between the threshold time (t) and the animal weight (W) and the related coefficient is 0.992.