Background & Aims: Considering the higher incidence of cardiovascular disorders in diabetes mellitus and some evidence on antioxidant and antidiabetic potential of Tribulus terrestris (TT), this study was conducted to evaluate the beneficial effect of 6-week oral administration of TT on contractile reactivity of isolated thoracic aorta in diabetic rats. Methods: Female Wistar rats were divided into control, TT-treated control, diabetic, glibenclamide-treated, and TT-treated diabetic groups. For induction of diabetes, streptozotcin (STZ) was administered (60 mg/Kg). Meanwhile, TT-treated groups received TT-mixed with standard pelleted food at a weight ratio of 6.25% for 6 weeks. Serum glucose level was measured at weeks 3 and 6. Finally, contractile reactivity of thoracic aortic rings to KCl and phenylephrine (PE) was determined. Results: Serum glucose level at weeks 3 and 6 showed a significant decrease in TT-treated diabetic group (P<0.01 and P<0.005 respectively) compared to diabetics. In addition, TT-treated diabetic group showed a significant lower contraction to PE (P<0.05) as compared to diabetic group and such significant reduction was also observed for KCl (P<0.05). Meanwhile, there was no significant difference between control and TT-treated control groups regarding their contractile reactivity to KCl and PE. Conclusion: Oral administration of TT for 6 weeks could exert a hypoglycemic effect and also attenuates the contractile responsiveness of the vascular system and this may prevent the development of hypertension in diabetic rats