Background: In the prediabetic stage, hyperinsulinemia or insulin resistnace is thought to be closely associated with oxidative stress which is the main contributor of endothelial dysfunction. Tea catechin has been reported to decrease oxidative stress and NADPH oxidase activity. The aim of this study was to investigate whether catechin could improve endothelial dysfunction and prevent hyperglycemia by improving insulin resistance and hyperinsulinemia in the prediabetic stage of a type 2 diabetic (T2D) rat model. Methods and Results: 20 Otsuka Long-Evans Tokushima Fatty (OLETF) rats at 13 weeks old, T2D rats, were randomized into 2 equal groups for treatment with either catechin (30 mg/kg) or saline for 12 weeks on an daily basis. Intraperitoneal glucose tolerance tests (IP-GTT) and BP measurements were performed at 13 and 25 weeks. Catechin significantly reduced BP (OLETF vs. Catechin-OLETF; 157 짹 8 vs. 136 짹 10 mmHg, p=0.005), fasting sugar (126 짹 4 vs. 103 짹 11 mg/dL, p=0.02) and insuline (2.1 짹 1.4 vs. 0.5 짹 0.3 ng/mL, p=0.004). In the thoracic aorta of Catechin-OLETF at 25 weeks, endothelial dependent relaxation and NADPH oxidase activity were markedly attenuated compared with that of catechin-OLETF. Catechin reduced vascular ROS formation in the thoracic aorta (dihydroethidium (DHE) staining). Catechin suppressed the expression of p47 phox and p22 phox (NADPH oxidase subunits) by immunohistochemical staining. Conclusion: Catechin reduced the formation of ROS and NADPH oxidase activity in T2D rat model, which resulted in improved endothelial dysfunction and insulin resistnace, and reduced BP.