Abstract: This study was carried out to detect hepatitis B surface antigen (HBsAg) and risk factors of transmission among children in katihar district of Koshi zone of Bihar State, India. In order to estimate the prevalence rate of HBsAg and to evaluate the influence of children demographics on HBsAg seropositivity, well-designed questionnaire was used to obtain data, considered risk factors for contracting HBsAg from consenting children. A total of 305 blood samples were collected from children attending the Katihar Medical College Hospital, Katihar. The male: female ratio was 1.38:1in the control group and 1.5: 1 in cases of jaundice. Blood samples were screened using HBsAg test kit, supplied by J. Mitra & Co. Pvt. Ltd, Delhi, India. HBsAg positivity rate among control was 3.5%. It gives fair idea about prevalence of HBsAg among healthy children of this region and does not indicate the carrier rate. The prevalence of HBsAg was higher among age group 9-12 years in jaundice patient. There is no significant difference in incidence of HBsAg positivity among males and females (p=0.317). Horizontal mode of transmission is found very significant. A higher rate of HBsAg positivity (17.64%) was found in patients of fulminant hepatic failure.