Objective To explore the protective effects of dynamin‑related protein 1 (Drpl)‑related mitochondrial fission and dimethylfumarate (DMF) by nuclear factor‑erythroid 2‑related factor 2 (Nrf2) on myocardial ischemia/reperfusion injury (MI/RI) in diabetic rats. Methods Healthy male SD rats were selected to prepare a diabetic model of rats. According to the random number table method, 60 diabetic rats were divided into four groups (n=15): a sham operation group (group S), a myocardial ischemia/reperfusion group (group MI/R), a DMF+myocardial ischemia/reperfusion group (group R), and a DMF+ML385+myocardial ischemia/reperfusion group (group RE). Group R and group RE received DMF 25 mg/kg gavage seven days before surgery, once a day, for consecutive seven days.Group S and group MI/R were given by gavage an equal volume of normal saline. Group RE was intraperitoneally injected with ML385 30 mg/kg 30 min before ischemia. The MI/RI model of diabetic rats was prepared by ligating the anterior descending branch of the left coronary artery for 30 min followed by perfusion for 120 min. Group S was only threaded but not ligated. Then, their heart rate, left ventricular systolic pressure (LVSP), left ventricular ejection fractions (LVEF) and left ventricular short axis shortening (FS) rate were recorded. The enzyme‑linked immunosorbent assay (ELISA) was used to measure the content of serum lactate dehydrogenase (LDH), creatine kinase MB isoenzyme (CK‑MB), and cardiac troponin I (cTnI). The activity of malondialdehyde (MDA), reactive oxygen species (ROS) content, and superoxide dismutase (SOD) activity in myocardial tissue were examined. The pathological change of myocardial tissues was detected by hematoxylin‑eosin (H‑E) staining. The size of myocardial infarction was observed by 2, 3, 5‑triphenyltetrazolium chloride (TTC) staining. The expression of myocardial Nrf2 and Drp1 mRNA was detected by reverse transcription polymerase chain reaction (RT‑PCR). The expression of myocardial Nrf2 and Drp1 was detected by Western blot. The morphological changes of myocardial mitochondria were evaluated by electron microscopy. Results Compared with group S, group MI/R, group R, and group RE showed decreases in heart rate, LVSP, LVEF and FS (P<0.05), increases in LDH, CK‑MB, cTnI, MDA and ROS content (P<0.05), decreases in SOD activity (P<0.05), myocardial pathological injury was aggravated,increases in myocardial infarction area (P<0.05), up‑regulation of Nrf2, Drp1 mRNA and protein expression (P<0.05), and increases in myocardial mitochondrial fission. Compared with group MI/R, group R presented increases in heart rate, LVSP, LVEF, and FS (P<0.05), decreases in CK‑MB, LDH, cTnI, MDA and ROS content (P<0.05), increases in SOD activity (P<0.05), myocardial pathological injury was alleviated, decreases in myocardial infarction area (P<0.05), up‑regulation of Nrf2 mRNA and protein expression (P<0.05), down‑regulation of Drp1 mRNA and protein expression (P<0.05), and decreases in myocardial mitochondrial fission. Compared with group R, group RE showed decreases in heart rate, LVSP, LVEF, and FS (P<0.05), increases in LDH, CK‑MB, cTnI, MDA and ROS content (P<0.05), decreases in SOD activity (P<0.05), myocardial pathological injury was aggravated, increases in myocardial infarction area (P<0.05), down‑regulation of Nrf2 mRNA and protein expression (P<0.05), and up‑regulation of Drp1 mRNA and protein expression (P<0.05), and increases in myocardial mitochondrial fission. Conclusion During the MI/RI of diabetic rats, DMF can stimulate Nrf2 to regulate Drp1‑related mitochondrial fission and exert its cardioprotective effect.