Abstract: Bangladesh is grappling with the largest mass poisoning in human history because groundwater used for drinking has been contaminated with naturally occurring inorganic arsenic (As). Special attention is necessary to pay to the children aged 10 years or more because they are in mid of physical growth as well as mental and social development. In this cross-sectional study, 130 junior high school students (14 and 15 years old) from Narayanganj district (highly As-contaminated area) of Dhaka division in Bangladesh were selected to investigate the effect of As exposure on intelligence quotient (IQ) and social competence (SC). Visiting each student’s house, information about the socio-economic status (SES) and drinking habits were collected. Urine samples were collected at school and brought back to Japan to analyze total arsenic concentration ([As]u), which was used as an index of As exposure. Student IQ was assessed with the Raven’s Standard Progressive Matrices test; while the Bengali version of Texas Social Behavior Inventory (TSBI) Form-A was employed for measuring SC. From the result, [As]u was significantly related with SES (especially household income). More important is that [As]u was significantly related with IQ and SC. The high [As]u group showed significantly lower IQ and SC values than the low [As]u group after SES variables were controlled. It was concluded that children’s IQ and SC were affected by chronic As exposure and these adverse effects may also gradually accumulate among the poor.