Mammalian fetal ovaries contains numerous primordial germ cells, however fewer ones can yield mature oocytes due to apoptosis and follicle atresia. Successful in vitro reconstitution of primordial germ cells has recently had a significant effect in the field of assisted reproductive technologies. However, the regulatory mechanisms underlying oogenesis remain unknown and recapitulation of oogenesis in vitro remains unachieved. Therefore, development of methods for obtaining mature oocytes by culturing the fetal ovaries in vitro could contribute to clarify these mechanisms. We adapt an in vitro system for culturing mouse fetal ovaries that support successful follicle assembly and improve oocyte growth and maturation. Ovarian tissues from 12.5 days postcoitum (dpc) fetal mice were cultured in vitro and the matured oocytes were differentiated from primordial germ cells after a 31 days culture period. Our results demonstrate that mouse fetal germ cells are able to form primordial follicles with artificial ovarian cells, and that oocytes within the growing follicles are able to mature normally in vitro. Taken together, this in vitro culture system is expected to aid in the development of new strategies to identify the reasons behind failure of follicle assembly and offer a platform for innovative research into preservation of female germ cells and conservation of endangered species.