Abstract: Objective To study the possible mechanism of radix morindae offieinalis（RMO） underlying myocardial ischemia-reperfusion injury（IRI） in rats by observing its effect on IRI. Methods Forty male rats were randomly divided into sham operation group,myocardial IRI model group,high-and low-dose RMO groups（10 rats in each group）.Rats in sham operation group and high-and low-dose RMO groups received a same volume of distilled water.Coronary artery in rats of sham operation group was not ligated,while the left anterior descending（LAD） of coronary artery in rats of the other 3 groups was ligated for 30min and reperfused for 60min.Then,their cardiac Infarction size,and creatine kinase（CK）,Na+-K+-ATPase,Ca2+-Mg2+-ATPase,nitric oxide（NO） and iNOS levels in myocardial tissue were measured. Results The cardiac infarction size was significantly smaller while the Na+-K+-ATPase,Ca2+-Mg2+-ATPase,NO and iNOS levels in myocardial tissue were higher and the CK level was lower in IRI model group than in high-and low-dose RMO groups（P＜0.05）. Conclusion RMO can protect myocardium against IRI in rats by preventing calcium overload and reducing oxygen free radicals.