Background: Coronary heart disease is a major cause of mortality and this health problem is reaching pandemic in both developed, and developing countries. ACS carries significant morbidity and mortality and the prompt diagnosis, and appropriate treatment is essential. HR was identified as a risk predictor of ACS. Both continuous increase in high baseline heart rate and decrease in low baseline heart rate are associated with higher risk of CVD. Decreased heart rate could also cause dispersion of atrial repolarization which, in turn, initiate cardiovascular events. Aim: The aim of this study was to assess the correlation between heart rate and severity of coronary artery disease in patient with acute coronary syndrome. Patients and Methods: The retrospective study was conducted on 120 patients that fulfilled the inclusion criteria were recruited from Cardiology department in Tanta university hospitals presented with acute coronary syndrome. Results: There was highly significant difference between both groups regarding admission heart rate & QTc interval. Both groups of low AHR< 60 bpm & high AHR >90bpm were significantly associated with severe CAD &.Mainly those patients with higher admission heart rate were more likely to have higher Syntax scores (severe coronary lesion). Also, patients with prolonged QTc had severe coronary artery diseases, higher SS & high probability to suffer adverse cardiac events more than patients without prolonged QTc interval. Conclusion: The current study showed that QTc interval prolongation and admission HR are independent predictors of the severity of coronary artery disease in patients with acute coronary syndrome.