Introduction: Magnesium (Mg) is the second most common intracellular cation after potassium (K). Mg is a cofactor in many enzyme systems in human cells and it has a predominant role in normal myocardial physiology. The role of magnesium in cardiovascular disease has received widespread attention. Magnesium has been implicated in the complications like arrhythmias in acute myocardial infarction. Objective: To know the serum magnesium level in patients with acute myocardial infarction and its relation with arrhythmia. Study design: Descriptive cross sectional study. Study place & period: Department of Cardiology, Rajshahi Medical College Hospital, Rajshahi from July, 2013 to November, 2013. Subjects: 50 patients with acute myocardial infarction admitted to the Department of Cardiology, Rajshahi Medical College Hospital. Methods: Data was collected from patients of any age and both sexes with acute myocardial infarction as determined by clinical features, ECG evidence and biochemical report. Blood sample for estimation of serum magnesium level was collected as early as possible within 24 hours of admission and 5th day of admission. After admission to CCU every patient was under continuous cardiac monitoring to see and record any arrhythmia within 5 days onset of symptoms. Results: A total of 50 patients of acute myocardial infarction were included during the study period. The male to female ratio in the study group was 3.17:1 and the maximum incidence of acute myocardial infarction was seen in 5th and 6th decade. The most common presenting symptom was chest pain which was present in all patients and was associated with sweat in 60% of patients and breathlessness in 64% of patients and palpitation in 50%. In the study, the most common risk factor found was smoking (70%) followed by diabetes (36%) and hypertension (30%). Anterior wall MI was found to be the most common type of MI (42%). Arrhythmia developed in (52%) patients and in majority (57.7%) of with anterior wall MI. In the study group mean serum magnesium level in 50 patients on day-1 is 1.86±0.39 and on Day-5 is 2.26±0.5. Mean serum magnesium level in 26 patients with arrhythmia was 1.65±0.26 on day-1 and 1.98±0.25 on day-5. In the study group, mean serum magnesium level in 24 patients without arrhythmia was 2.05±0.41 on day-1 and 2.48±0.52 on day-5. The difference between the magnesium level in patients with arrhythmia and without arrhythmia is statically significant on both day- 1 and day-5 (p<0.001). PVC was the most common type (42.5%) of arrhythmia. Conclusion: serum magnesium levels are significantly low in patients who develop arrhythmia in acute myocardial infarction.