Objectives: to study the relation between atrial fibrillation (AF) and in-hospital outcome in patients with acute coronary syndrome (ACS) who were treated by primary percutaneous coronary intervention (PCI). Methods: This study was conducted on 80 patients admitted with ACS and treated with primary PCI at cardiovascular medicine department Tanta university hospitals starting from January 2020 till January 2021. The primary end points are all cause mortality and major adverse cardiovascular events (MACE) including a composite of death, nonfatal re-infarction, target vessel revascularization (TVR), new onset congestive heart failure, contrast induced nephropathy (CIN), or stroke during hospitalization. Patients was divided into 2 groups: Group 1: consisted of 40 consecutive AF-patients treated by primary PCI. Group 2: consisted of 40 consecutive sinus rhythm-patients treated by primary PCI. Results: Patients in AF group showed significantly older age, lower systolic and diastolic blood pressure, higher heart rate, higher Killip class II-IV, more inferior STEMI presentation, higher CK-MB, more RCA as infarction related artery, more moderate to sever mitral regurgitation, more patient developed congestive heart failure during hospitalization, and higher overall MACE during hospitalization. Univariate and multivariate regression analysis were performed to investigate the possible predictors of AF in the study population. In univariate regression analysis, older age, higher CKMB level, higher degree of mitral regurgitation, enlarged left atrium, and RCA as infarction related artery were correlated with AF. In the multivariate regression analysis, using model adjusted for aforementioned parameters, older age, higher CK-MB level, enlarged left atrium diameter, and RCA as infarction related artery independently predicted AF. Univariate and multivariate regression analyses were performed to investigate the possible predictors of overall in-hospital MACE in the study population. In univariate regression analysis, smoking, Killip II-IV, high creatinine level, lower ejection fraction, higher end systolic diameter, and AF were correlated with MACE. In the multivariate regression analysis, using model adjusted for aforementioned parameters, Killip II-IV, higher creatinine level, and AF independently predicted MACE. Conclusion: Patients older in age, with higher CK-MB level, enlarged left atrial diameter, and RCA as infarction related artery had higher incidence of AF during ACS. Patient with AF who presented with ACS had a higher incidence of heart failure during hospitalization. The independent predictors of MACE in our study were AF, Killip II-IV, and higher creatinine level.