Hepatitis B virus (HBV) prevalence has declined remarkably in children due to nationwide universal vaccination program for HBV in China. However, the persistence of immune response against HBV infection and the optimal time point when a booster vaccination should be performed remain to be elucidated. To assess the persistence and level of antibody against hepatitis B surface antigen (anti-HBs) in a representative population of age 15 and younger who received routine hepatitis B vaccination in Mianyang City, China. A cross-sectional study was conducted in 2011. One thousand five hundred twenty-six children of age 15 and younger who received three doses of 5 μg hepatitis B vaccine series during infancy but did not receive a booster vaccination later were enrolled. Of the 1,526 children, the mean age was 8.2 ± 4.1 and 739 children were male. The median anti-HBs level was 23.0 mIU/mL, and the total percentage of anti-HBs levels ≥10 mIU/mL was 60.9%. With an increase of age, median anti-HBs level, percentage of anti-HBs levels ≥10 mIU/mL, and percentage of anti-HBs levels ≥100 mIU/mL declined remarkably in the early period and reached the lowest level at the age of 3 and then remained relatively stable. The median anti-HBs level, the percentage of anti-HBs levels ≥10 mIU/mL, and the percentage of anti-HBs levels ≥100 mIU/mL in 1- and 2-year-old children were much higher than that in children aged 3-15 (p < 0.05, respectively). Immunity against HBV infection gradually decreased in early ages of children of 15 and younger who received three doses of 5 μg hepatitis B vaccine series during infancy in China. Three dosages of 10 μg hepatitis B vaccine for infants and repeated vaccination or additional booster vaccination for some children at or before age 3 should be provided to get much more powerful immunity to HBV.