Abstract : Thalassemia is the inability of the bone marrow to synthesize hemoglobin. Blood transfusion and chelation therapy is the treatment for beta thalassemia major. However, hyperkalemia is a common side effect because stored blood has elevated potassium level. In light of the significance of blood transfusion therapy in beta thalassemia major patients and the risk of hyperkalemia, this study was carried out in to investigate the effect of transfusion therapy on the changes in potassium levels, particularly in beta thalassemia major pediatric patients. This research was an observational paired numerical analysis conducted to evaluate the effect of blood transfusions on plasma potassium levels in beta thalassemia major pediatric patients at Hematology Oncology Haji Adam Malik Hospital. Blood sampling to test the plasma potassium levels was carried out before and one hour after transfusion. Spearman correlation test was used to determine whether blood storage and blood transfusion volume affected potassium level in the plasma. Paired T-test and Wilcoxon tests were used for the correlation analysis. Most research subjects (32/44, 72.7%) had increased plasma potassium level after transfusions. The mean potassium levels increased from 3.80 mEq/l to a mean value of 5.8 mEq/l after blood transfusions. There was a difference in potassium level between before and after transfusion (p<0.001). The volume of transfused blood did not significantly affect potassium level (p = 0.597, r = -0.082). Similarly, the storage time also did not show significant relationship with potassium level (p = 0.509, r = -0.102). The increase in potassium level in beta thalassemia major was not correlated with the volume and storage time of blood transfused.