Aim To evaluate the effect of trimetazidine on myocardial stunning by pressure-volume loops in rats and its probable mechanism. Methods Thirty SD rats were randomly divided into 3 groups equally,control group and myocardial stunning group （physiological saline 2 mL）, trimetazidine group （trimetazidine tablet 3 mg/kg）. All rats, except for those in control group, were subjected to 20 minutes of left anterior descending artery occlusion and 120 minutes reperfusion, while in control group only threading without ligation. Hemodynamic variables （as heart rate, left ventricular end-diastolic pressure, left ventricular end-systolic pressure and so on） were dynamicly observed by the pressure-volume loops and PowerLab system and software offline were applicated to analyse. Myocardial tissue ATP content, ATPase activity and phosphate fructokinase （PFK） activity were detected after reperfusion for 120 min, and myocardial mitochondria ultrastructure was tested using the electron microscope by stereology method. Results Compared with myocardial stunning group, in trimetazidine group, end-diastolic pressure（EDP）, end-systolic volume （ESV） and preload recruited stroke work （PRSW） were significantly reduced （P<0.01）, end-diastolic pressure-volume （EDPVR） were also decreased （P<0.05）. End-diastolic volume （EDV） and end-systolic pressure-volume （ESPVR） were obviously increased by pressure volume ring analysis. ATP content and ATPase activity elevated （Ca²⁺-Mg²⁺ATPase and Na⁺-K⁺ATPase, P<0.05） PFK activity added（P<0.01） and myocardial mitochondria ultrastructure significantly ameliorated abnormal changes （P<0.01）. Conclusions Trimetazidine attenuates myocardial stunning by the energy metabolism, and pressure volume ring can evaluate cardiac function accurately and sensitively.