Periodontal infections are related to the expansion of diabetes cardiovascular problems. However, the pathological process and probable mechanism remain unexplained. This study investigated the impact of periodontitis on streptozotocin (STZ)-induced diabetes rats' carotid artery. We randomized 24 Sprague-Dawley (SD) rats into four groups: control, chronic periodontitis (CP), diabetes mellitus (DM), and DM +CP groups. Fasting blood glucose (FBG) and hemoglobin A1c (HBA1c ) were measured to verify the establishment of the DM model. After euthanasia, the maxillary was collected for further studies like hematoxylin-eosin (HE), Masson staining, and micro-computed tomography (micro-CT) analysis. Immunofluorescence (IF) staining was used to detect endothelial-mesenchymal transition (EndMT)-related markers in carotid artery wall. We further used ELISA and quantitative real-time PCR to investigate the effect of high glucose (HG) and Porphyromonas gingivalis lipopolysaccharide (P.g-LPS) on human umbilical vein endothelial cells (HUVECs). Compared with DM and CP groups, bone resorption and pathological changes of the vascular wall were the most serious in the DM+CP group. The vascular wall of the DM+CP group had a higher level of interleukin (IL)-6 and vascular cell adhesion molecule 1 (VCAM-1). The carotid artery vascular wall of the DM+CP group contained more cells that expressed both mesenchymal and endothelial cell markers, along with elevated transcription factor levels. Furthermore, P.g-LPS and HG upregulated the inflammatory cytokines expression and caused phenotypic changes of HUVECs in vitro. Periodontitis exacerbates endothelial dysfunctions partly via endothelial-mesenchymal transition in STZ-induced diabetes rats.