Aim To investigate the electrocardiographic features and its possible mechanisms at the moment of myocardial ischaemia and reperfusion injury (IRI) in patients with acute ST segment elevation myocardial infarction (STEMI). Methods The electrocardiographic changes during reperfusion of acute STEMI by thrombolysis or percutaneous coronary intervention (PCI) were analysed. Effective data of IRI electrocardiographic pattern were explored with univariate analysis. 60 cases of acute STEMI were classified into myocardial reperfusion with IRI electrocardiographic changes group (reperfusion group A,n=39) and without IRI electrocardiographic changes group (reperfusion group B,n=21). The levels of serum reactive oxygen species (ROS),malondialdehyde (MDA),reduced glutathione (GSH),total antioxidative capacity (T-AOC) and γ-glutamylcysteine synthatase (γ-GCS ) activity were measured in control group (n=43),myocardial ischaemia group (before reperfusion therapy,n=60),reperfusion group A and reperfusion group B. Results Among the 60 cases of acute STEMI,39 patients (65.00%) with IRI arrhythmia (accelerated idioventricular rhythm 26.67%,non-paroxysmal ventricular tachycardia 10.00%,sinus bradycardia 10.00%,and atrioventricular nodol block 8.33%)as well as 28 patients (46. 67%) with IRI ST segment elevation were observed. Univariate analysis revealed that the duration from chest pain to receiving reperfusion therapy,the peak time of CK-MB ,and reperfusion time were shorter in the reperfusion group B than those in the reperfusion group A (P><0.05). The more IRI electrocardiographic changes were detected during reperfusion treated by thrombolysis than by PCI (P><0.05). The levels of serum ROS and MDA in the reperfusion group A were significantly higher,but the GSH and T-AOC were significantly lower than those in the control group,myocardial ischaemia group,and reperfusion group B (P><0.05). The γ-GCS activity in reperfusion group A was significantly higher than that in the control group and myocardial ischaemia group (P><0.05). Conclusion This study confirms the occurrence of specific electrocardiographic changes (IRI arrhythmia and ST segment elevation) at the time of reperfusion. Systemic oxygen free radicals oxidative/antioxidative imbalance occurs at the moment of reperfusion in patients with acute STEMI,which may correlate to IRI electrocardiographic changes.