Objective: To study the relationship of the mutation of HBV preS/S gene in HBsAg carrying pregnant women and intrauterine transmission. Methods: Polymerase chain reaction (PCR) was used to amplify HBV preS/S gene from sera of 8 HBsAg carrying pregnant women, 4 women’s neonates infected with HBV, and the other’s neonates non-infected with. The PCR products were cloned and 5 clones were chosen from every woman for DNA sequencing. Results: Heterogeneity of HBV preS/S gene in HBsAg carrying pregnant women having intrauterine transmission was much higher than that from having not intrauterine transmission, and the divergence rate of nucleotide sequences also higher strikingly. Conclusion: High heterogeneity of HBV preS/S gene of HBsAg positive pregnant women may be relative to high rate of intrauterine transmis- Objective: To study the relationship of the mutation of HBV preS / S gene in HBsAg carrying pregnant women and intrauterine transmission. Methods: Polymerase chain reaction (PCR) was used to amplify HBV preS / S gene from sera of 8 HBsAg carrying pregnant women, 4 women’s neonates infected with HBV, and the other neonates non-infected with. The PCR products were cloned and 5 clones were chosen from every woman for DNA sequencing. Results: Heterogeneity of HBV preS / S gene in HBsAg carrying pregnant women having intrauterine transmission was much higher than that from having not intrauterine transmission, and the divergence rate of nucleotide sequences also higher strikingly. Conclusion: High heterogeneity of HBV preS / S gene of HBsAg positive pregnant women may be relative to high rate of intrauterine transmis- sion