Abstract: Objective By observing the placental ultramicrostructure in thefetuses infected by HBV in uterus from their mothers, to investigate the cellular mechanism of HBV transmitting. Methods HBV was determined in mother' serum,cord serum and neonatal serum with ELISA. Five patients'placentae ultramicrostructure of which HBV was transmitted to the fetuses were observed with JEM-1200EM. Results The HBV transmitting rate from mother to fetus in HBV antigens both positive women was 60%. The changes of HBV transmitting placentae include the increasing of ribosome and shape changes of microvilli, the trophoblastic necrosis, the capillary basal lamina in placenta barrier thickness with fibrin-like substance deposits and the exsistance of HBV-like particles in cell spaces, plasma and nuclears. Conclusions HBV can be transmitted from mother to fetus in uterus by infecting and duplicating in the syncytial trophoblast.