Objective: To investigate the influence of body mass index (BMI) and its change on the incidence of type 2 diabetes mellitus (T2DM) in an adult cohort. Methods: A total of 3 043 subjects randomly selected among those without T2DM at baseline survey from the Rural Deqing Health Cohort Study were included into this study and follow up was conducted for 1 867 of them from July to November 2015. The subjects were divided into 3 groups according to their baseline BMI, normal group (BMI<24.0), overweight group (BMI: 24.0-27.9) and obese group (BMI≥28.0) and the subjects with stable weight (BMI change of-0.05 to + 0.05 per year) were used as the referent category. Cox proportional model was used to estimate associations between BMI, its changes and T2DM. Results: With an average of (8.68±1.25) person years, 213 new T2DM cases, including 125 females were detected, and incidence density was 13.14 per 1 000 person years. After adjusted for other covariates, the risk for incidence of T2DM was significantly higher in subjects with baseline BMI≥28.0 kg/m2 than those with baseline BMI<24.0 kg/m2 (aHR=2.12, 95% CI: 1.22-3.68), and weight gains were strongly associated with the incidence of T2DM in those with baseline BMI< 24.0 kg/m2 (aHR=2.21, 95%CI:1.17-4.17). Conclusion: Both BMI and its change were significantly associated with incidence of T2DM, especially in the population with BMI<24.0 kg/m2 at baseline.