Results of the studies on the morphologic and molecular biologic characteristics of Hantaan virus (HTNV), one of the etiologic agents of Hemorrhagic fever with renal syndrome (HFRS), revealed that HTNV was a member of Family Bunyaviridae and its RNA divided into three segments. And the nucleotide sequences of these segments also were known and the differences in nucleotide sequences of HTNV from other members of genus Hantavirus were clearly evaluated. But the morphorgenesis, pathogenesis of HFRS and the replication time had not been clearly determined. In this study, to estimate the replication time of HTNV in Vero E-6 cells, Vero cells were infected with HTNV 76/118 strain, and cells were harvested from two hours post-infection up to 24 hours at two hours-intervals. Harvested cells were treated with ordinary techniques for electron microscopy and immune-electron microscopy. And then thin sections were observed under transmission electron microscope. HTNV particles were not found in the cytoplasm and in the extracellular space between $2{sim}8$ hours after inoculation of virus, but virus particles were observed in extracellular space near the cell membrane of Vero-E6 cells 10 hours after infection. In immune electron microscopy, mature HTNV particles in extracellular spaces and immature virus labelled with gold particles in the cytoplasm of Vero E-6 cell 10 hours after infection of HTNV could be seen. This results suggest that the replication time of HTNV might be about 10 hours.