Background: Endothelial dysfunction is a critical and initiating factor in the pathogenesis of diabetic vascular complication. Krüppel-like factor 2 (KLF2) is implicated as a key molecule regulating endothelial vascular function. We evaluated the reciprocal role of a forkhead transcription factor, FOXO1, and HMG-CoA reductase inhibitor, atorvastatin, in KLF2 regulation in diabetic condition. Methods and results: In human umbilical vein endothelial cells (HUVECs) incubated in high glucose condition, FOXO1 was activated whereas KLF2 and eNOS expression was near completely abolished. Causal relationship was confirmed by FOXO1 adenoviral transfection which significantly suppressed KLF2 expression and by FOXO1 siRNA which reversed glucose-induced suppression of KLF2 and eNOS. Such induction of FOXO1 with high glucose was blocked by atorvastatin not only by increasing phosphorylation but also by accelerating degradation of FOXO1 through Akt-mediated ubiquitination. The ChIP analysis of the promoter region of KLF2 confirmed that glucose or atorvastatin modulated FOXO1 activity on KLF2 transcription. In the vessels harvested from the OLETF rats, the animal model of type II diabetes, FOXO1 was activated, whereas KLF2 and eNOS expression were depleted, which was significantly reversed by atorvastatin. Atorvastatin also significantly restored impaired endothelium-dependent vasodilation in the OLETF rats at the same range of improvement as KLF2 replenishment. Conclusion: KLF2 suppression by FOXO1 in endothelial cells might be a plausible mechanism of diabetic endothelial dysfunction. High glucose-induced, FOXO1-mediated KLF2 suppression was reversed by atorvastatin. These findings suggest implication of intensive statin treatment in preventing diabetic vascular dysfunction.