Objective:To investigate the relationship between β－cell function and fasting plasma glucose. Methods:History taking,physical examination,laboratory tests,75－g oral glucose tolerance tes(OGTT) and insulin secretion test were performed on 1 068 sujects without confirmed diabetes from Gaoyou district and Nanjing University of Science and Technology community of Jiangsu Province. Venous blood samples were collected at 0,30 and 120 min of OGTT to measure plasma glucose and serum insulin. MatsudaISI and HOMAIR were used to determine insulin sensitivity. The area under curve (AUC) of insulin during 0 to 30 min and 0 to 120 min(AUCINS30 and AUCINS120),together with the ratio of AUC of insulin to the AUC of glucose(i.e. insulin release indices,including INSR30 and INSR120) were calculated as surrogate indices of β－cell insulin secretion function. To evaluate the compensatory response of β－cell to insulin resistance,we used the products of insulin release indices multiplied by Matsuda ISI as disposition indices (DI30 and DI120). Results:When fasting plasma glucose (FPG) level was in the rang of 4.6~5.6 mmol/L,INSR30 and INSR120 decreased significantly. AUCINS30 and AUCINS120 decreased significantly when FPG≥7.0 mmol/L. Matsuda ISI always attenuated along with FPG increased,while HOMAIR was enhanced. Interestingly,DI30 and DI120 decreased steadily even at the normal glucose. After grouped by BMI,AUCINS30,AUCINS120,INSR30 and INSR120 of obese subjects were larger than non－obese,but the insulin sensitivity was less than non－obese. There was no difference between obese and non－obese subjects on DI30 and DI120. Conclusion:β－cell function was already impaired when the FPG was in high normal range,although the sum of insulin secretion is not reduced,the compensatory response of β－cell to insulin resistance has already decreased.