Aim To investigate the protective effect of Crocin on myocardial mitochondria in rats with acute myocardial infarction (AMI) and its mechanism. Methods 150 SD rats were randomly divided into Sham group, AMI group, Crocin (7.5,5, 30 mg/kg) group according to random number table method (n＝30). The AMI rat model was established by ligating the left anterior descending coronary artery. 24 h later, the ultrastructure changes of myocardial mitochondria in ischemic area was observed by transmission electron microscope; the membrane potential and the opening of mitochondrial permeablity transition pore (MPTP) were detected by fluorescence spectrophotometer; the myocardial mitochondrial respiratory function index (R3, R4, RCR) were detected by oxygen electrode method. The respiratory enzymes activity, ATP content were detected by colorimetry method; the Na⁺-K⁺-ATPase, Ca²⁺-ATPase activity were detected by phosphorus determination method; the mitochondrial Ca²⁺ concent was detected by atomic chemiluminescence. ResultsCompared with AMI group, the ultrastructural lesions such as mitochondrial swelling, membrane rupture, crest rupture and dissolution were significantly improved in Crocin 5,0 mg/kg groups, the membrane potential increased and MPTP openness decreased (P＜0.01); the R3, RCR increased and R4 decreased (P＜0.05 or P＜0.01); respiratory enzymes (NADH dehydrogenase, succinate dehydrogenase, cytochrome C oxidase) activity, ATP content and Na⁺-K⁺- ATPase, Ca²⁺-ATPase activity increased (P＜0.05 or P＜0.01), Ca²⁺ concentration decreased (P＜0.05 or P＜0.01). Conclusion Crocin has protective effect on the structure and function of myocardial mitochondria in the ischemic area of AMI rats, which mechanism may be related to inhibiting the increase of mitochondrial Ca²⁺ concentration.