Objective To determine the factors responsible for failed postnatal immunoprophylaxis for hepatitis B virus(HBV) in Qidong, China. Methods Eleven children who developed into chronic HBV infection after receiving HBIG and HBV recombinant vaccines were recruited into the study. Eleven paired mothers with chronic hepatitis and other 6 mothers whose children successfully generated anti-HBs after im-munoprophylaxis were included as the control in the study. Full-length HBV DNA was amplified through ser-um sample by PCR method and underwent cloning and sequencing. HBV DNA level was quantified by real-time PCR. Results The mean levels of HBV DNA in mothers who had HBV DNA positive children and healthy children were ( 1.2 ×107± 3.1 × 106 ) copies/ml and ( 1.6× 107±8.8×106 ) copies/ml, respec-tively. There was no significant difference between the groups (P >0.05). Meanwhile, viral load in chil-dren was unrelated to that in their mothers (r2 =0.2429). In 11 HBV DNA positive children, 4(36.4% ) demonstrated amino acid substitutions in HBsAg a determinant region with 6 different types, I.e. T125A, I126T, Q129H, M133V, D144V and G145A. All of the mothers showed the wild-type sequence in a epitope, indicating surface escape mutants were not acquired from the initial infection, but developed under the immune pressure. The mutation rates after immunoprophylaxis for preS1, preS2, S, X, preC/C and P genes were 0.38%, 0. 22%, 0.27%, 0.17%, 0.11%, and 0.11%, respectively, nt2999-3157 in preS1, nt529-677 in S, nt1955-2016 in C, nt923-1001 and nt2489-2602 in P genes were among the hottest muta-tional spots throughout the HBV genome. Conclusion HBV mutation may occur in all the open readingframes after passive and active immunoprophylaxis. In addition to S gene, HBV preS and P genes could alsoassociate with the escape mutants. Key words: Hepatitis B virus; Immunoprophylaxis; Vertical transmission; Mutation