Background: Transfusion dependent thalassemia (TDT) patients will progressively experience iron overload due to repeated blood transfusions. It also causes vitamin D hydroxycholecalcoferol (25OH) depletion in the liver parenchyma. Methods: This was a cross-sectional study of children with TDT in Hemato-oncology outpatient clinic, Dr. Soetomo Hospital Surabaya which has enrolled in the study from January-june 2020. Serum ferritin level by Enzym linked Fluorescent Assay (ELFA) and vitamin D hydroxycholecalciferol (25OH) level was examined by the ADVIA centaur VitD assay method. To determine the correlation between serum ferritin level and vitamin D hydroxycholecalciferol (25OH) Spearman correlation was used, significant if p<0.05 Results: There were 11 boys and 19 girls with TDT, their age ranged between 7-18 years old. The mean value of serum ferritin level was 2570 (SD 1805.6) ng/ml, and serum vitamin D hydroxycholecalciferol (25OH) was 16 (SD 8.06) nmol/ml. 26/30 subjects had serum ferritin level > 1000 ng / ml, 12/30 subjects were considered deficient in vitamin D hydroxycholecalciferol (25OH), with vitamin D hydroxycholecalciferol (25OH) level <15 nmol/ml. The serum ferritin level had a negative correlation with vitamin D hydroxycholecalciferol (25OH) level (r = -0.476; p <0.008). Conclusion: Increased serum ferritin followed by decreased vitamin D hydroxycholecalciferol (25OH) in children with TDT