Objective To investigate the impact of diabetes mellitus (DM）and increasing body mass index (BMI) category on carotid vascular structure and elastic function assessed by the quality intima-media thickness (QIMT) and quality arterial stiffness (QAS) techniques of high-frequency ultrasound, and observe their correlation. Methods 90 diabetic patients and 90 non-diabetic sex and BMI matched control subjects were enrolled in this study. The two groups were then divided into three subgroups based on the BMI category. High frequency ultrasound QIMT and QAS technology were used to determine the intima-media thickness (IMT), stiffness index (β), pulse wave propagation velocity (PWV) and compliance coefficient (CC) of common carotid artery in all subjects. Also, the correlations between elastic parameters of common carotid artery parameters were analyzed. Results In the same BMI categories, the values of carotid IMT, PWV and β in DM patients were significantly higher than those in NON-DM subjects, while CC was lower than that in NON-DM patients (P<0.05). Also, carotid IMT, PWV and β in both DM and control group were increasing with BMI, and CC was decreasing with BMI. On multivariate linear regression analysis, both DM and obesity were independently associated with an additive detrimental effect on carotid intima thickness and vascular elasticity. Interestingly, obesity seems to be more associated with carotid dysfunction than DM. Conclusion Both DM and increased BMI have additive adverse effects on carotid vascular structure and vascular elastic function. Furthermore, increasing BMI classification was associated with greater carotid dysfunction than DM. Future clinical studies on DM should simultaneously focus on the obesity.