Abstract: Objective Obesity is closely related to the occurrence and development of type 2 diabetes mellitus. Body mass index (BMI) is a worldly recognized classification method for assessing the degree of obesity. To investigate the relationship between body mass index (BMI) and hyperglycemia and evaluate the diagnostic value of BMI in predicting diabetes in the first-degree relatives of type 2 diabetic patients. Methods We conducted a prospective cohort study in a sample of 296 first-degree relatives without glucose intolerance history. All adults were examined on height, weight, calculated BMI and underwent an oral glucose test (OGTT). Their level of fasting blood glucose, 30-minute postprandial plasma glucose, 60-minute postprandial plasma glucose, 120-minute postprandial plasma glucose were determined by the glucose oxidaseperoxidase method. The area under the blood glucose curve (AUCg) was calculated by the trapezoidal rule. Evaluation of the value of BMI levels in diabetes screening by receiver operating characteristic curve analysis (ROC). Results (1) The prevalence rates of impaired glucose regulation and diabetes were 30. 07% and 19. 93% in the first-degree relatives of type 2 diabetic patients respectively. From normal glucose tolerance condition through impaired glucose regulation to diabetic, the BMI progressively increased (P < 0. 001). (2) Area under the curve sugar of OGTT (AUCg) as dependent variable, age and BMI as the independent variable in normal glucose tolerance group, linear stepwise regression analysis showed that age and BMI were the independent factors (P=0. 015). (3) The optimal cutoff value of sensitivity and specificity of BMI were 24. 95 kg/m², 67. 3% and 62. 2% by receiver operating characteristic (ROC) curve. Conclusion BMI is a strong predictor for hyperglycemia in the first-degree relatives of type 2 diabetic patients. It is therefore suggested that BMI ≥ 24. 95 kg/m² be recommended as diagnostic cut-off points in the first-degree relatives of type 2 diabetic patients.