Fully epitaxial germanium-on-insulator structures have been grown for the first time on (001) Si substrates by using a perovskite oxide template. Detailed transmission electron microscopy analysis revealed that a two-temperature growth procedure is required in order to avoid three-dimensional island growth and to obtain Continuous and flat Ge films. The interface between the oxide and Ge is fully crystalline and atomically sharp. A high density of stacking faults and twin structures has been found preferentially originating from the surface toughness of the oxide.