β-cell dysfunction plays a prominent role in type 2 diabetes (T2D) etiology, and obesity is a major cause of insulin resistance and T2D. This study aimed to assess the different roles of β-cell dysfunction and insulin resistance in the progression from normal glucose tolerance to T2D in obese and non-obese Chinese people. A total of 3254 participants aged ≥ 25 years, including 1,843 healthy controls and 1,411 newly diagnosed T2D patients were recruited. All participants were categorized into the non-obese control, obese control, non-obese T2D and obese T2D groups. The obese subjects had higher homeostasis model assessment of insulin resistance and β-cell function (HOMA-IR and HOMA-β) than the non-obese ones, whether in the control group or the T2D group (all P <0.01). Higher HOMA-IR and lower HOMA-β were observed in non-obese T2D patients when compared with non-obese controls (all P <0.01). The obese T2D group had lower HOMA-β than the obese control group (P< 0.01). There was no significant difference in HOMA-IR between the obese T2D and obese control groups. The decline of β-cell function, together with persistent insulin resistance, accounts for the progression from normal glucose tolerance to T2D in obese people, while the decline in β-cell function mainly contributes to the progression in non-obese people.