For group 14 mono-elemental 2D materials, such as silicene, germanene, and stanene, oxidation is a severe problem that alters or degrades their physical properties. This study shows that the oxidized germanene on Ag(111)/Ge(111) can be reformed to germanene by simple heating ≈500 °C in a vacuum. The key reaction in reforming germanene is the desorption of GeO and GeO