Background: Beta-thalassemia major is a type of inherited blood disease that results in variable outcomes such as severe anemia due to haemoglobin chains. Recurrent and lifelong blood transfusions as a treatment in beta-thalassemia major disease lead to iron deposition in various organs and cause the failure of multiple organs. Failure of affected organs leads to Body mass index (BMI) abnormality. This study aimed to evaluate the association between BMI and serum ferritin level as a marker for iron overload. Materials and Methods: A cross-sectional study designed and conducted with total number of 740 paediatrics, with mean age about 14.2±8.7 years old and with beta-thalassemia major requiring recurrent blood transfusion. Patient information, including demographics, serum ferritin level and percentage of BMI, was recorded and analysed by SPSS 25.0 and the statistical significant level, considered as 0.05. Results: A total number of 740 paediatrics with beta-thalassemia major disease (mean age about 14.2±8.7 years) were included to study to examine the association between serum ferritin level and their BMI. The total mean serum level of ferritin calculated about 3326 ± 3859 Nanogram/mililitter (ng/ml). Totally, 447 (60.4%) case of them had BMI percentile less than 5%, 274 (37.02%), 16 (2.16%) and 3 (0.4%) had BMI percentile 5%-85%, 85%-95% and more than 95%. There was no relation between gender and serum ferritin levels. The relationship between age and BMI has been positive (P=0.002). Finally, it resulted that there was a negative relationship between the BMI percentile and mean serum ferritin levels in paediatrics with beta-thalassemia major (P=0.031). Conclusion: Frequent Blood transfusion is associated with elevated serum ferritin level in paediatrics with beta-thalassemia major disease and experiencing lower percentiles of BMI in these patients.