Abstract: AIM To investigate the effects of Mdivi-1, a small molecule inhibitor of dynamin-related protein 1 (Drp1), on myocardial ischemia-reperfusion injury in diabetic mice. METHODS High-fat diet and streptozotocin-induced diabetic mice were subjected to myocardial ischemia-reperfusion (MI/R) or sham operation. Mdivi-1 (1.2 mg/kg) or vehicle (dimethyl sulfoxide) was administrated to the mice 15 min before the onset of reperfusion by intraperitoneal injection. Outcome measures included mitochondrial morphology, cardiac functions, myocardial injury and apoptosis. The expression of Drp1 was analyzed by Western blot. RESULTS Enhanced mitochondrial fission and increased mitochondrial Drp1 translocation were induced by I/R in diabetic hearts. Delivery of Mdivi-1 into diabetic mice markedly inhibited Drp1 translocation to the mitochondria and reduced mitochondrial fission following MI/R. Inhibition of Drp1 reduced myocardial infarct size, inhibited cardiomyocyte apoptosis and alleviated oxidative stress in diabetic MI/R mice. CONCLUSION Drp1-mediated mitochondrial fission is involved in diabetic MI/R injury. Pharmacological inhibition of Drp1 with Mdivi-1 prevents mitochondrial fission and reduces I/R injury in diabetic mouse hearts.