AIM:To explore the effect of HbA1c level in the patients with type 2 diabetes mellitus (T2DM) on the rheological properties of erythrocytes and the structure of hemoglobin (Hb).METHODS:The patients with T2DM were classified into 3 groups:the patients with good glycaemic control (group A, HbA1c<7.0%), the patients with poor glycaemic control (group B, 7.0%≤HbA1c<9.0%) and the patients with persistent hyperglycemia (group C, HbA1c≥9.0%) .The rheological properties of a single living erythrocyte were analyzed by the techniques of static imaging and anal -ysis.Ultraviolet-visible spectroscopy was used to study the structure of Hb .RESULTS: Compared with group H , the roundness effect factor (REF) of erythrocytes, which was positively related with HbA1c level, was significantly increased in groups A, B and C (P<0.05).The contact area of erythrocytes in group B and all the morphological parameters of erythrocytes in group C were significantly higher than those in group H ( P <0.05 ) .There were significant differences among groups A , B and C in deformation capacity and elastic parameters of the cell membrane ( P<0.05 ) , but with no difference in the long axis and short axis between group A and group B .Compared with group H , no obvious change in the spectral pattern and spectrum peak of Hb in groups A , B and C was observed.However, the absorbance of Hb, which showed a trend of gradual decline with the increase in HbA 1c level, in group B and group C was significantly decreased as compared with group H (P<0.01), and the Hb absorbance in group C were also lower than that in group A (P<0.05). CONCLUSION:With the increase in HbA1c level, the morphology along with the deformation function of erythrocytes in T2DM changes and declines gradually , and the structure of Hb may also change .