Thalassaemia patients are the highest consumers of blood. Recurrent exposure to allogenic red cell antigen puts this population at increased risk of alloimmunization. This causes delayed hemolytic transfusion reactions. So transfusion requirement increases. But no data regarding alloimmunization was available in Bangladesh. Aim of this study was to estimate the prevalence of alloimmunization and to find out the potential factors associated with its development. This analytical cross sectional study was done by enrolling 97 patients, received at least 10 units transfusions, through convenient sampling. Indirect and direct antiglobulin tests were done to detect immunization by spin tube technique keeping an autocontrol and carried out in the Department of Haematology & Department of Transfusion Medicine, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh from March 2016 to March 2017. Polyclonal anti-AHG reagent was used. A total 21 patients developed alloimmunization (21.6%) and 4 patients (4.1%) developed autoimmunization. Age, gender, splenectomy and number of transfusion are shown significant risk factors for alloimmunization. Data from this study demonstrate that the RBC alloimmunization is significantly high in our country. So, pretransfusion antibody screening needs to be initiated in order to ensure safe transfusion and RBC phenotyping should be started before starting first transfusion to prevent alloimmunization.