Objective:To study the relationship of hyperglycemia and the cognitive function of patients with type 2 diabetes mellitus (T2DM).Methods:Questionnaires were used to study cognitive function of 135 T2DM patients. Pre- and postprandial and pre-bed seven-point glucoses (fingerstick) were collected. The differences of cognitive function among the different glucose level groups were analyzed. Results:① Fasting plasma glucose (FPG)≥6.1 mmol/L group and <6.1 mmol/L group had prominent difference in the total scores,orientation and composition of a picture of mini-mental state examination (MMSE),the second body regions immediate memory,the first immediate auditory verbal and pictorial recognition(P < 0.05).②There were significant differences between the postprandial glucose(PPG)≥10.0 mmol/L group and <10.0 mmol/L group on the wrong numbers of the cancelling test,naming picture and motor testing.③In the highest glucose group(the 7-point-daily capillary glucose≥10.0 mmol/L),the rapid verbal retrieve2 (animal),the second body regions immediate memory,the delayed body regions memory,the clued body regions memory and digital span had lower scores than those in <10.0 mmol/L group. ④When the median of mean blood glucose (MBG) (8.7 mmol/L)acted as cut point,compare to <8.7 mmol/L group,≥8.7 mmol/L group has bad performance in the orientation and composition of a picture of MMSE,the delayed auditory verbal,the clued auditory verbal and naming picture.⑤There was no significant difference between the two FPG,PPG group,the highest glucose group and MBG group on sex,age,duration of diabetes,educational level,combined hypertension or not and blood lipid. HbA1c had no effect on cognitive function in PPG group,the highest glucose group and MBG group,but had effects on cognition in the two FPG groups. ⑥FPG,PPG,the highest glucose of the 7-point-daily capillary glucose and MBG were negatively correlated with cognitive function. Conclusion:Hyperglycemia may aggravate the cognitive dysfunction of T2DM. patiewts with FPG ≥6.1 mmol/L or PPG≥10.0 mmol/L coule have cognitive lesion. Controlling hyperglycemia and reducing the glucose fluctuation effectively will be beneficial to preserve cognitive function of T2DM patients.