Abstract: Objective To determine whether exendin-4 (Ex-4)might improve endothelial dysfunction in aorta isolated from high-fatty diet induced obese rats. Methods 20 male Wistar rats were divided into normal control and high-fat supplement (OB) groups (n=10 for each group). The rats were sacrificed after 10 weeks feeding and thoracic aorta was dissected and cut into four rings of 3 mm length, the response to acethylcholine (Ach) and sodium nitroprusside (SNP) were examined in organ bath containing Krebs-Henseleit (K-H) solution. In order to study the direct effects of Ex-4 on obese rats vascular function, the aortic rings obtained from obese rats were incubated with K-H solution (OB-C group, n=10),or with K-H solution plus Ex-4 (OB-Ex group, n=10). Aortic rings obtained from normal control group (n=10) were incubated with K-H solution. After 1 hour of incubation, the aortic rings were precontracted with norepinephrine (0.1 μmol/L), then the rings were exposed to cumulative concentration of Ach (10^-9-10^-4 mol/L)or sodium SNP (10^-10-10^-6 mol/L) to test the endothelial dependent (EDV) and independent vasodilation (EIV). The blood plasma of the rats was collected for biochemical test and celiac fat and body mass data were also obtained. Results The levels of serum triglyceride, total cholesterol, and free fat acid were elevated in the obese group compared with that of normal control group (P<0.01). The ratio of celiac fat and body mass in the obese rats was also higher than the control (P<0.05). Ach caused a concentration dependent vascular relaxation in all pre-constricted aortic rings. Compared to the control group, maximal endothelium dependent relaxation in the obese group was impaired (P<0.05). The EIV values were comparable between two groups. Pre-incubation of obese rat's vessels with Ex-4 significantly increased cumulative relaxation to Ach (P<0.05). SNP induced vessels relaxation had no statistical significance each groups (P>0.05). Conclusion Endothelial function was impaired in obese rats. Ex-4 directly mitigates impaired endothelial-dependent vasorelaxation of isolated obese rat's aortas.