Introduction: Thalassemia is the most common hereditary disorders worldwide, including Indonesia. It is proven to be a burdening disease. Chronic anaemia and iron overload in thalassemia major lead to several risk factors including cognitive problems, but there are only few studies reported about cognitive function in children with thalassemia major. There has been no research about cognitive functions in children with thalassemia major in Thalassemia Center of Indonesia. Purpose: (1) To investigate cognitive function in children with thalassemia major (2) to identify factors related to cognitive function in children with thalassemia major (age at diagnosis, duration of transfusion, patients education, pre-transfusion haemoglobin level, ferritin, transferrin saturation, compliance to chelation, and electroencephalography test). Materials and Methods: A cross-sectional descriptive analytic study conducted from April 2016 to April 2017 on Thalassemia Center Cipto Mangunkusumo Hospital. Intelligence quotient assessed using Wechsler intelligence scale for children test-III (WISC-III). Electroencephalogram (EEG) results were analyzed by a pediatric neurology consultant. Subjects were excluded if they had epilepsy, central nervous system (CNS) disorders, or serious comorbidities. Results: A total 70 subjects with thalassemia major aged 9 to 15.5 years old performed IQ test. The prevalence of abnormal IQ score (<85) was 37.1%. Six per 70 subjects were mild mentally retarded and 2/70 subjects were moderate mentally retarded. Mean (Â±SD) of total IQ score was 88.8 (Â±16.91). Subjects in this study had abnormal IQ score prevalence 2.7 times greater and experienced an average IQ score decrease by 11.2 points than the normal population. Sixty percent of EEG results were abnormal generalized background slowing activity. A correlation existed between the thalassemia patients IQ and pre-transfusion Hb level (p<0.05). Conclusion: Our study found high prevalence of abnormal IQ test in children with thalassemia major. Pre-transfusion Hb level needs to be maintained between 9 and 10 g/dL to prevent cognitive dysfunction. Further study is suggested to explore the factors related to cognitive function in children with thalassemia major.