Objective: To evaluate the effect of intravenous magnesium sulfate on post-operative complications when given prophylactically while coming of cardio pulmonary bypass (CPB) in patients undergoing cardiac surgery. Methodology: A double-blinded randomized studyconducted at a tertiary care center in western Maharashtra from Nov 2017 to April 2019. Patients were divided into two groups. Group A (Magnesium sulfate group) (n=130) & Group B (control group) (n=130). Patients undergoing on pump Coronary artery bypass grafting (CABG), Mitral valve replacement/repair (MVR), Aortic Valve Replacement (AVR) or both AVR & MVR (DVR) on CPB were included in the study.Once surgery was over, aortic cross clamp taken out & temperature of 36°c was attained, Group A patients received Injection Magnesium sulfate 1 gm IV slowly over 10 minutes.Patients were monitored for heart rate, intra-arterial blood pressure oxygen saturation, continuous electrocardiograph with automatic ST segment analysis apart from this ECG mapping was done. Results:There was no significant Statistical difference regarding the demographic data, comorbidities & ejection fraction between the two groups. The result shows a statistically significant difference (P=0.032) between the two groups with regards to the incidence of atrial fibrillation. In group A 10 patients (7.70%) had atrial fibrillation compared to group B where 28 patients (21.6%) had atrial fibrillation.Weaning from CPB was easier in group A patient compared to group B & also group A patient needed smaller doses of pharmalogical support, than group B. Conclusion: A single bolus 1Gm dose of magnesium sulfate while coming off from CPB has an excellent effect in reducing the incidence of atrial fibrillation. Further magnesium reduced the amount of pharmacological support which was required to come off CPB.