Endothelial dysfunction caused by oxidative stress plays a key role in atherogenesis. This study investigated whether the anti-diabetic drug miglitol, an α-glucosidase inhibitor, which is currently available in clinical practice, can prevent endothelial cell apoptosis and whether it might restore impaired vascular relaxation under oxidative stress. The bEnd.3 cells, a microvascular endothelial cell line, were pre-treated with various concentrations of miglitol and then were incubated with H₂O₂ for 1 – 2 h. Treatment of bEnd.3 cells with miglitol resulted in the protection of cell viability, suppression of mitochondrial superoxide production, and DNA strand breakage under the oxidative stress. These effects of miglitol were associated with the activation of AMP-activated protein kinase (AMPK) and the phosphorylation of endothelial nitric oxide synthase (eNOS). In aortic rings with endothelium, acetylcholine (Ach)-induced relaxation was attenuated by H₂O₂. We found that this impaired relaxation was restored by acute treatment with miglitol. Compound C, an AMPK inhibitor, inhibited the amelioration of vascular relaxations treated with miglitol. These results suggest that miglitol might protect against endothelial cells damage under oxidative stress via inhibition of endothelial cell apoptosis and mitochondrial superoxide production, which are mediated by the activation of AMPK and the phosphorylation of eNOS.