Objective： We investigated the association between body mass index (BMI) and risk of major adverse cardiovascular events (MACE) in people with diabetes. Methods： We analyzed data from a National Health Insurance Service - National Health Screening Cohort (NHIS-NHSC). Diabetes was defined by the ICD-10 numbers E10-E14. We used standard BMI definitions for underweight (< 18.5 kg/㎡), normal weight (18.5-22.9 kg/㎡), obese class I (25.0-29.9 kg/㎡) and obese class II ( > 30 kg/㎡). Cox’s proportional hazard regression model was used to estimate hazard ratio (HR) and its 95% confidence interval (95% Cl) for risk of MACE. Age, gender, systolic blood pressure, festing glucose, total cholesterol, smoking status, alcohol consumption, physical exercise, socioeconomic status, and using of antihypertensive drugs, lipid-lowering drags, anti-platelet drags, and glucose-lowering drugs were adjusted as potential confounders. Results: A total of 40,673 people with diabetes include in the analysis. During a median of 10.5 years of follow-up (interquartile range, 10.1-11.2), the observed numbers of MACE (total observed person-time) were 6,633 (403,482 person-years). Compared to normal weight people, adjusted HRs (95% Cl) for MACE were 1.12 (0.95-1.3D for underweight people, 1.01 (0.95-1.08) for overweigh people, 1.01 (0.95-1.08) for obese class I people, and 1.02 (0.90-1.15) for obese class II people. When classified to subtypes of MACE, underweight people had higher risk of cardiac death (HR = 1.15； 95% Cl： 1.10-2.03) and hospitalization for myocardial infraction (HR = 1.53; 95% Cl： 1.16-2.02) compared to normal weight people. Compared to normal weight people, adjusted HRs for unstable angina (95% Cl) was 1.29 (1.13-1.46) and 1.19 (0.92-1.54) in overweight and obese class I people, respectively. Conclusion： Our findings suggested that BMI was not associated with overall risk of MACE. However, underweight people had higher risk of cardiac death and myocardial infarction, and overweight and obese people had higher risk of unstable angina compared to normal weight people.