The purpose of the study was to determine whether impaired beta-cell function exists in Chinese individuals within the normal range of glucose tolerance (NGT), and these individuals are predisposed to diabetes later in life. The cross-sectional study included 843 NGT subjects and 562 isolated impaired glucose tolerance (IGT) patients and the longitudinal study included 1,724 NGT subjects. Insulin secretion was assessed using indices derived from oral glucose tolerance test and adjusted by insulin resistance. NGT subjects were sub-divided into two groups: NGT-l (2hPG<125 mg/dl) and NGT-h (2hPG 125-140 mg/dl). Normal weight subjects were individuals with BMI<25 kg/m2, and overweight were with BMI≥25 kg/m2. In normal weight subjects, the first- and second-phase insulin secretion indices were significantly higher in NGT-h and NGT-l subjects compared with IGT subjects. However, in overweight subjects, first-phase insulin secretion index in NGT-h subjects was significantly lower than that in NGT-l subjects, but similar to that in IGT patients. The second-phase insulin secretion was comparable between NGT-h and NGT-l subjects. After an average follow-up of 43.80±11.25 months, totally 25 (1.5%) NGT subjects at baseline developed diabetes. The incidence rate of diabetes was higher in NGT-h overweight subjects (9.2%) than in NGT-l overweight subjects (1.5%) with a risk ratio (RR) reaching 6.655 [95% confidence interval (CI) 2.347-18.867]. This risk remained after adjustment for sex, age, BMI, systolic pressure, and diastolic pressure (RR 8.315, 95% CI 2.649-26.108). It is concluded that overweight NGT adults with high-normal 2hPG (≥125 mg/dl) had a defect in first-phase insulin secretion and were with the increasing risk for developing new diabetes.