Introduction: Mitral valve disease is a common heart condition that may require mitral valve replacement (MVR) surgery. One of the potential complications after MVR is the development of atrial fibrillation (AF). Left atrial volume index (LAVI) is a measure of left atrial size that has been linked to the development of AF in patients with mitral valve disease. The present study aimed to observe the post-operative support needed by patients after MVR surgery, particularly those with elevated LAVI and the incidence of post-operative AF. Methods: This prospective observational study was conducted at the Department of Cardiac Surgery, National institute of cardiovascular diseases (NICVD), Sher-e-Bangla Nagar, Dhaka, Bangladesh. The study duration was 1 year, from Mar, 2018 to Feb, 2019. During this period, a total of 60 patients who underwent Mitral Valve Replacement (MVR) for mitral valve diseases at the study hospital were included in the study following the inclusion and exclusion criteria through a purposive sampling method. The 60 patients were divided in two equal groups, Group A, consisting of 30 Patients who had Left Atrial Volume Index of ≥ 39 ml/m2 after operation, while group B had been comprised of 30 patients who had Left Atrial Volume Index of < 39 ml/m2 after operation. Result: Around 23.33% of participants in both groups were aged 21-30, while 40% of Group A and 36.66% of Group B were aged 31-40. In terms of BMI, 53.33% of Group A and 46.66% of Group B had a normal BMI, while 40% and 43.33% were overweight, and 6.66% and 10% were obese, respectively. Specifically, 23.33% of participants in Group A developed post-operative AF, while only 6.66% of participants in Group B developed it. The p-value for this comparison was 0.015. The results demonstrate that Group B had significantly smaller LA diameter, LA volume, LAVI, LVIDS, and LV ejection fraction compared to Group A at all three measurements post-operation (p<0.05). However, there was no significant ...