Objective To study the vasodilating effect of the acetone- extract (AE) from Cortex Mori (CM) and its vasodilating mechanism. Methods Guinea pig models with contraction of mesentery blood capillary induced by noradrenalin were used to study the in- vivo vasodilating effects of AE from CM. In- vitro effect of AE from CM on rat thoracic aortic ring was observed. The contents of nitric oxide (NO), nitric oxide synthase (NOS), constitutive NOS(cNOS) and inducible (iNOS) in plasma and aortic tissue were determined by method of nitroreductase chromatometry. Results AE from CM had vasodilating effect on the contracti on of mesentery blood capillary in guinea pigs and the contraction of rat thoracic aortic ring induced by 10? mol/L phenylephrine, even under the condition of pre- treating with glibenclamide (1 ? mol/L) or propranolol (3 ? mol/L). When the endothelium of the rat thoracic aortic ring was removed, AE from CM had not vasodilating effect but constrictive effect. AE from CM could also increase the contents of NO, NOS and cNOS in the aortic tissue and had no obvious effect on the NO content in plasma and iNOS in aortic tissue of rats. Conclusion The vasodilating mechanism of AE from CM may be related to promoting the release of NO from endothelium of blood vessel and synthesis of NOS and cNOS in aortic tissue.