Background and Aims: Diabetes mellitus is accompanied with higher incidence of cardiovascular disorders. There is some evidence on antidiabetic potential of silymarin. In this study the effect of chronic administration of silymarin on contractile-relaxation response of thoracic aorta of diabetic rats was investigated. Methods: In this experimental study, male Wistar rats were divided into 5 groups of control, silymarin - treated control (100 mg/kg), diabetic, and silymarin -treated diabetic groups (50 and 100 mg/kg). Ten days after streptozotocin injection, silymarin was daily administered (i.p.) for 4 weeks. At the end of study, contractile reactivity of thoracic aortic rings to KCl and phenylephrine and relaxation response to acetylcholine were determined using isolated tissue setup. Results: Silymarin-treated diabetic group showed a significantly lower maximum contraction to KCl (at high dose) and phenylephrine at both doses (p<0.05-p<0.01) as compared to the diabetic group. Maximum relaxation response of rings to acetylcholine was significantly higher in silymarin-treated diabetic group (high dose) as compared to diabetics (p<0.05). Meanwhile, there was also a higher relaxation response in silymarin-treated control group (high dose) in comparison to controls (p<0.05). Conclusion: Chronic administration of silymarin could decrease contractile response and enhance relaxation response in aortic tissue of diabetic rat and this may be beneficial in prevention of long-term vascular complications of diabetes.