Rationale and Background: : Hepatitis B virus (HBV) vaccine has been developed in early 1980s and shown good efficacy. However, children with HIV infection had poor immune response to many kinds of vaccines. Objectives: : To assess the persistence of vaccine immunity against HBV after the primary vaccination in infancy and the response to revaccination in HIV-infected children in Mahosot hospital. Methodology: : This study was prospectively conducted in HIV-infected children at Mahosot hospital from September 2021 to August 2022. Seventy HIV-infected children were enrolled. Inform consent was performed before starting the study. Anti-HBs was checked, if anti-HBs≥10 mIU/mL, the subjects would not receive booster dose for Hepatitis B vaccine. If anti-HBs<10 mIU/mL, the patient will receive one booster dose and checked for anti-HBs 4-8 weeks after. If anti-HBs was still<10 mIU/mL, another 2 booster doses will be given and anti-HBs is checked 4-8 weeks after. Results: : We enrolled 70 HIV-infected children, most of them were older than 10 years (median=12.5). The mean age at diagnosis of HIV was 2.9 years. Sixty-seven (95.7%) had anti-HBs titer <10 mIU/mL at baseline (first visit). After the first booster, 18(26.9%) had anti-HBs titer ≥10 mIU/mL. In 49 subjects who had anti-HBs <10 mIU/mL after the first dose and received another 2 booster HBV doses, 10(20.4%) still had anti-HBs <10 mIU/mL. Anti-HBs titers at baseline, after 1st booster and after 3rd booster were5.2, 8.2, and 30mIU/mL, respectively. Conclusion: In HIV-infected children who received primary series of HBV vaccine, almost all of them (95.7%) had anti-HBs lower than seroprotective level (<10 mIU/mL). Only one fourth achieved seroprotective level after the 1st booster but 79.6% achieved seroprotective level after the 3rd booster. The results suggested that HIV-infected children should receive 3 doses of HBV revaccination.