Context. Clinical studies demonstrated erythrocyte deformability (ED) is impaired in diabetic patients and described the correlations between HbA1c and ED. Few studies further investigated what an exact elevated HbA1c level linked to the impairment of ED in diabetes. Objective. This study was to determine a cutoff point of HbA1c level leading to the impairment of ED in patients with diabetes. Design. This was a retrospective observational study. ROC curve analysis was used to determine an optimal cutoff value of HbA1c for the increasing HSRV. Subjects and Methods. In this study, 300 type 2 diabetic patients were enrolled. The whole blood viscosity was measured. High shear reductive viscosity (HSRV) was used to indirectly estimate ED. Based on the obtained cutoff value and glycemic control criteria for HbA1c, we divided all the cases into different groups to further confirm the accuracy of the cutoff value. Results. In 300 patients, ROC curve illustrated that 9.05% was the optimal cutoff value as a predictor of the increasing HSRV. And higher odds ratio (OR) for significant decrease in ED was seen in the patients with HbA1c >9.05% compared to those with HbA1c≤9.05% (OR: 3.78, 95% CI: 2.08-6.87). HSRV increased significantly in patients with HbA1c level >9.05% in comparison to patients with HbA1c levels <6.5% between 6.5 and 8.0% and between 8.0 and 9.05%. Conclusion. ED decreased significantly in diabetic patients as soon as HbA1c level was higher than 9.05%.