OBJECTIVE This study aims at detecting C gene mutation strain in vertical transmission of hepatitis B virus (HBV). METHODS HBV C gene nt 2,022 - 2,321 was directly sequenced. RESULTS Homology of HBV nucleotide sequence and amino acid sequence in mothers, fathers and their fetus was 100%. Mutation of 2,189 and 2,288 nucleotide of C gene resulted in substitution at the 97th and 130th amino acids. CONCLUSION C gene mutation strain is present in vertical transmission of HBV. Presence of mutation strain may be the cause of failure in vaccination.