Abstract： Objective The purpose of this study was to explore characteristics of beta cell function and insulin resistance in subjects with impaired fasting glucose (IFG). Methods A total 3 985 subjects (2 664 males, 1 321 females), aged 20～89 years, were studied. 75 g oral glucose tolerance test was performed after a 10 h overnight fasting, and fasting insulin (FINS), 2 h insulin (2 hINS) levels were measured. These subjects were divided into six groups: normal glucose tolerance (NGT)( n =2 588), impaired fasting glucose (IFG) ( n =272), impaired glucose tolerance (IGT) ( n =449), IFG/IGT ( n =116), newly diagnosed diabetes mellitus (DM1) ( n =338), known diabetes mellitus (DM2) ( n =222). We evaluated their insulin secretion and insulin resistance by IS=FINS/FPG, HBCI=20×FINS/(FPG 3.5), and Homa IR=(FPG×FINS)/22.5, and WC,BMI,SBP,DBP,TC,TG and HDL C. Results Homa IR was higher in subjects with IFG, IGT and IFG/IGT than that in subjects with NGT (1.46±0.60, 1.39±0.58, and 1.70±0.61 vs 1.06±0.64, P = 0.001 ), but there was no significance between IFG and IGT( t=-1.303,P =0.193). Subjects with IFG had higher FINS level than NGT (17.90±10.06 vs 15.79±10.94, P =0.039). There was no significance between IFG and NGT ( P =0.072) for 2 hINS. HBCI and IS were lower in subjects with IFG than subjects with NGT and IGT(HBCI 4.65±0.60 vs 5.27±0.76 and 5.47±0.79, P <0.01), (IS 0.86±0.6 vs 0.99±0.62 and 1.25±0.61, P=0.04, P <0.001, respectively). Conclusion The subjects with IFG might have hepatic insulin resistance and defect of insulin secretion. The IGT might have peripheral insulin resistance and high insulin secretion. Both IFG and IGT presented as clinical features of the metabolic syndrome. IFG/IGT showed significant defect in insulin secretion. Compared with IFG and IGT, the defect in insulin secretion of IFG/IGT approached diabetes mellitus.