OBJECTIVE： To study the vasodilatory effect of oxysophocarpine （OSC） on isolated thoracic aortic rings of rats and its possible mechanism. METHODS： Thoracic aortic rings of rats were collected （called “vascular ring” for short）. Using K-H nutrient solution as blank control and the diastolic rate as index， the effects of different concentrations （0.2-1.0 mg/mL） of OSC on normal vascular rings in basal state， normal or endothelium-free vascular rings pre-contracted by norepinephrine （PE， 1×10－6 mol/L） were investigated. After pre-culturing normal thoracic aortic rings by nitric oxide synthase inhibitor L-nitro-arginine methyl ester（L-NAME）and cyclooxygenase inhibitor indomethacin（INDO），as well as pre-culturing endothelium-free vascular rings by potassium ion channel blocker BaCl2，tetraethylammonium（TEA）and 4-aminopyridine（4-AP）， the diastolic effects of OSC of different concentrations （0.2-1.0 mg/mL） on the above vascular rings were investigated by using the same method. RESULTS： Compared with blank control， there was no significant effects of different concentrations of OSC on the diastolic rate of normal vascular rings in basal state （P＞0.05）， but 0.4-1.0 mg/mL OSC could significantly improve the diastolic rate of normal or endothelium-free vascular rings pre-contracted by PE （P＜0.01）， in concentration-dependent manner. After preculturing with L-NAME， INDO， 4-AP and BaCl2， different concentrations of OSC had no significant effect on the diastolic rate of normal or endothelium-free vascular rings pre-contracted by PE （P＞0.05）. After pre-culturing with TEA and Gli， 0.4-1.0 mg/mL OSC could significantly reduce the diastolic rate of endothelium-free vas- cular rings pre-contracted by PE （P＜0.01）. CONCLUSIONS： OSC did not significantly dilate the thoracic aortic rings of rats in the basal state within the dose range （0.2-1.0 mg/mL）， but OSC of 0.4-1.0 mg/mL have significant diastolic effects on the normal or endothelium-free thoracic aortic rings of rats pre-contracted with PE. The mechanism of thoracic aortic rings dilation is endothelium-independent， which may be associated with receptor operational calcium channel，Ca2+-activated potassium channels and ATP-sensitive potassium channels.