AIM: To investigate the role of the insulin secretion as well as insulin sensitivity in the onset and development of the type 2 diabetes mellitus (T2DM). METHODS: This study included 38 normal glucose tolerance(NGT) subjects without family history of diabetes, 32 NGT and 36 impaired glucose tolerance(IGT) subjects who were the first-degree relatives of type 2 diabetes, and 35 newly-diagnosed type 2 diabetic patients. ? cell function and insulin sensitivity were examined by using oral glucose-insulin release test (OG-IRT) with SIM reflecting insulin sensitivity and intravenous glucose tolerance test (IVGTT) with AIR3-5 reflecting the first-phase insulin secretion function. AIR3-5 was calculated as the average incremental plasma insulin concentration from the third to the fifth minute after the glucose bolus. AIR3-5 and SIM were compared among the four groups. RESULTS : From NGT, IGT to type 2 diabetes, there was a decreasing tendency of the level of AIR3-5 and SIM, with NGT the highest, IGT the medium and T2DM the lowest. The level of the AIR3-5 in the subjects of NGT who were first - degree relatives of type 2 diabetes were lower than those of NGT without family history of diabetes( P 0.05) . CONCLUSION: Insulin secretion dysfunction may be the primary factor in the onset of type 2 diabetes. Along with the developing of IGT and type 2 diabetes, the insulin secretion and sensitivity decrease.