Abstract: Bovine preantral follicles (about 100μm in diameter) were cultured in vitro in a serum -free system for 15 day. The transmission electron microscope analysis showed that the preantral follicles isolated mechanically were 1 to 3 layers of granulose cells tightly enclosing the oocyte and intact basement membrane without theca cell. There are obvious and intensive gap junctions between the adjacent granulose cells. The ultrastructural characteristics of the preantral follicles isolated freshly or cultured in vitro were mostly similar to those presented in situ. The proliferatating of granulose cells was observed on the sixth day of culture. The theca interna cell emerged in a few preantral follicles on the 15th day of culture. These results showed that the culture system is adapted for smaller preantral follicles’ growing.