Background Atrial fibrillation (AF) is a common complication of acute myocardial infarction(AMI) and have been linked to an increased risk for in-hospital and long-term mortality rates. Electrocardiogram (ECG) parameters which correlate with occurrence of AF were not evaluated except P wave duration and P dispersion. We sought to evaluate the association between ECG parameters and occurrence of AF in patients with acute ST elevation myocardial infarction (STEMI). Methods We retrospectively analyzed the association between ECG parameters and the incidence of AF in a total of 100 patients with STEMI. Resting ECG parameters including heart rate, P wave duration, PR interval, QRS duration, QTc interval, ST elevation and Q/QRS ratio, were measured from ECGs on admission. 24-h Holter monitoring was performed 6짹2 days after STEMI. Information about occurrence of AF was collected from Holter monitoring and daily serial ECGs. Results Paroxysmal new-onset AF occurred 20(20%) of the patients with STEMI. Subjects with higher level of P wave duration have high risk of AF development (p=0.001) Heart rate, PR interval, QRS duration, QTc interval, ST elevation and Q/QRS ratio had no relation with occurrence of AF in patients with STEMI. In multilinear and logistic regression models to determine independent predictor of AF, P wave duration independently predicted the occurrence of AF in patients with STEMI (p=0.001). Conclusion Among the resting ECG parameters, prolonged P wave duration was an important risk factor for AF in the patients with STEMI.