Background/aim: Diabetes mellitus (DM) is a major health problem worldwide. Cinnamic acid (CA) and its derivatives are synthesized in plants and increasing attention has been given to them in recent years due to the high number of beneficial health properties attributed to their consumption. The aim of this study was to investigate the effects of CA on streptozotocin-induced diabetes in Wistar albino rats. Materials and methods: DNA damage was evaluated in the blood, liver, and kidney cells of rats by the alkaline comet assay. Oxidative stress parameters such as catalase, superoxide dismutase, glutathione reductase, glutathione-S-transferase, and glutathione peroxidase activities and 8-hydroxy-2-deoxyguanosine, total glutathione, and malondialdehyde levels; biochemical parameters including insulin, total bilirubin, and BCA protein levels; hepatic enzyme levels such as alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, and gamma-glutamyl transferase; and lipid profile parameters including high-density lipoprotein, low-density lipoprotein, total cholesterol, and triglyceride levels were also evaluated. Results: DM caused genotoxic damage and alterations in lipid profiles, oxidative stress parameters, and hepatic enzymes levels. CA treatment ameliorated these effects. Conclusion: It seems that CA might have a role in the prevention of the complications of diabetes.