SCOPE: Endothelial dysfunction is a critical factor during the initiation of diabetic cardiovascular complications. Polyphenols may represent beneficial dietary components eliciting cardiovascular protection. Although we previously reported that the polyphenol morin (MO) ameliorated diabetes-induced endothelial dysfunction, the underlying mechanism remains unclear. Here, we investigated protective effects and mechanisms of MO in streptozotocin STZ induced diabetic aorta endothelial dysfunction. METHODS AND RESULTS: Diabetes was induced by tail vein injection of STZ (200 mg/kg). At 12 wk after injection, the thoracic aorta was isolated and endothelial function was assessed by acetylcholine (ACh) induced, endothelial-dependent vasorelaxation in aortas. Nitric oxide (NO) levels and endothelial NO synthase (eNOS), phosphorylated-eNOS (p-eNOS), Akt, and phosphorylated-Akt (p-Akt) levels were also evaluated in aortas. Diabetic aortas showed attenuated endothelial function, which was improved by MO treatment. MO treatment alone increased NO levels and endothelial-dependent relaxation responses via Akt signaling, although ACh did not activate this pathway. Moreover, MO upregulated p-Akt (at Ser473 and Thr308) and p-eNOS (at Ser1177) expression in diabetic aortas, but ACh stimulation had no effect on p-Akt and p-eNOS levels. CONCLUSION: These results indicate a novel role for MO in protection against endothelial dysfunction in diabetes. The protective effects of MO are dependent on Akt-dependent activation of eNOS signaling.