Background: Hepatitis B virus (HBV) infections is moderately endemic in many countries in the sub-Saharan Africa including Tanzania. Immunization of children below five years of age has been found to be an effective strategy in controlling infectious diseases. However, the data regarding immune responses following vaccination are very limited in low-income countries. Here, we report the sero-conversion among children below five years of age after three doses of HBV vaccine in Mwanza, Tanzania. Methodology: A cross-sectional study involving children below five years of age was conducted at Makongoro Reproductive and Child Health (RCH) clinic between May and June 2017. Sociodemographic data were collected, and vaccination status was confirmed from reproductive and child health (RCH) cards. Serum HBV surface antibodies (anti-HBs) were quantified using enzyme immunoassay (Enzygnost Anti-HBs II). Data were analysed by using STATA version 13 software.