||Nanoparticles of nickel and iron are prepared by inert gas condensation of plasma evaporated vapour. The atomic structures of Ni and α-Fe nanoparticles have been investigated by high-resolution transmission electron microscopy (HRTEM). It is found that the Ni core is protected by a pyramidal hillock NiO shell with 3.6nm and 2.4nm in maximum and minimum height respectively; the α-Fe core is protected by γ-Fe2O3 with the average thickness of 4nm epitaxy oxide shell from further oxidation at room temperature. The following epitaxial relationship of core-shell metal-oxide in Ni and α-Fe have been observed. They are (111)NiO // (111)Ni , [11̄0]NiO // [1̄10]Ni and (111)NiO // (001)Ni , [11̄0]NiO // [1̄10]Ni for Ni. (001)γ-Fe2O3 // (001)α-Fe , α-Fe2O3 // [l00]α-Fe for α-Fe. The protection of the metal core of nanoparticle by few nanometer oxide layer at 295K (room temperature) can be understood from the Caberra-Mott theory of oxidation of metal surface.