Abstract: OBJECTIVE To study the protective effects and the mechanisms of Crataegi Folium flavonoids (CFF) on acute myocardial ischemia induced by isoproterenol in rats. METHODS The 48 healthy male Sprague-Dawley rats were randomly divided into normal control group, positive control group, model group and the group of CFF(50, 100, 200 mg·kg^-1). Myocardial ischemia model of rats were induced by subcutaneous injection of isoproterenol(10 mg·kg^-1 for each time, once a day, and lasted for 2 days). The change of electrocardiogram(ECG) was observed. The levels of lactate dehydrogenase(LDH), cmatine kinase(CK) in blood plasma, the activity of superoxide dismutase(SOD), catalase(CAT), and the content of malondialdehyde(MDA) in myocardial tissue were measured. The changes of myocardial morphology were observed as well. RESULTS Compared with the model group, CFF(50, 100, 200 mg·kg^-1) could effectively fight against ST-segment elevation induced by isoproterenol, notably decrease serum LDH and CK enzymic activity, distinctly increase the levels of CAT and SOD in myocardial tissue(P<0.01 or P<0.05), significantly decrease the content of MDA in myocardial tissue, and relieve morphological changes of myocardial. CONCLUSION CFF has a protective effect on acute myocardial ischemia induced by isoproterenol in rat, the protective mechanisms of CFF may be related to anti-oxidation.