Atomic force microscope (AFM) is a powerful and prospective tool in the studies of the cellular deformability within a single cell. However, the reliability of using AFM in estimating the cellular average deformability was suspected. Hemorheology, one of common clinical examinations, can assess the deformability of erythrocytes. In order to validate the reliability in estimating the cellular deformability with AFM, the correlation was studied between the stiffness of the erythrocyte membrane with AFM and the deformation index with the hemorheology test. Blood samples were taken from 5 diabetes mellitus patients and 5 healthy non-obese people. The erythrocyte deformability was detected by AFM and the hemorheology test, respectively. Excellent correlation (r=0.907, p=0.000<0.01) was found between the average erythrocyte stiffness with AFM and the deformation index obtained from the hemorheology test. Besides, it was found that the average stiffness and the deformation index of diabetes mellitus patients were significantly higher than those of healthy non-obese people (p=0.007 and p=0.003). The result was consistent with the impairment of erythrocytes form diabetes mellitus patients. Therefore, it is the first time that the quantification relation of the erythrocyte deformability was investigated with AFM and the hemorheology test, and the feasible and reliable were validated that AFM is used to investigate the mechanical properties of different living cells qualitatively and quantitatively.