Ni matrix nanocomposites dispersing Cr nanoparticles have been developed by using electrodeposition.The nanocomposites, in comparison to the Ni matrix composites dispersing Cr microparticles,exhibits the exceptional ability of formation of a protective chromia scale against high temperature oxidation at 900 ℃ in air.The fundamental reason is interpreted on a basis of XPS depth profile and TEM characterization of the initial oxide scale formed on the Ni-Cr nanocomposite.At the onset of oxidation, the nanoparticles in the surface layer act as the "diffusionless" nucleation sites for formation of chomia islands and those in deeper zone quickly dissolve in the Ni matrix and diffuse to the surface for the growth of the chromia islands.