Background: Cinnamaldehyde has anti-diabetic effects such as blood glucose and lipid regulation, insulin resistance improvement, and antioxidation. However, there have been few related reports published on the effect of cinnamaldehyde in the treatment of diabetic vascular endothelial dysfunction and prevention of diabetic vascular complications. We aimed to explore the effect and mechanism of cinnamaldehyde on glucose metabolism and vessel function in db/db mice. Material/Methods: General condition of mice (n= 10 for each group) such as food intake, fasting blood glucose, body weight, and insulin sensitivity was monitored periodically during the intervention time for 12 weeks. Immunofluorescent staining and hematoxylin and eosin staining of the pancreas were employed to observe the effects of cinnamaldehyde on the function and morphology of pancreatic islets. Acetylcholine (Ach) solution was used to observe Ach-induced endothelium-dependent vasodilatation and nitroglycerin (NTG) solution was used to observe NTG-induced endothelium-independent vasodilatation. Results: There was significant improvement in general condition of db/db mice, including food intake, fasting blood glucose, body weight, and insulin sensitivity. After cinnamaldehyde intervention, the function and morphology of pancreatic islets was significantly improved in db/db mice compared to the control group. The level of superoxide anion significantly decreased while the level of nitric oxide significantly increased in db/db mice. Cinnamaldehyde had significant effects on endothelium-dependent diastolic function and vascular remodeling. Conclusions: Cinnamaldehyde can significantly reduce fasting glucose levels, increase insulin sensitivity, and improve islet morphology and function in db/db mice. Experiments showed that cinnamaldehyde could effectively improve vascular endothelium-dependent diastolic function with antihypertensive trend, which provides experimental evidence for further cinnamaldehyde clinical use.