The electronic structure of nickel nanoparticles encapsulated in carbon was characterized by photoelectron, X-ray absorption, and X-ray emission spectroscopies. Experimental spectra were compared with the density of states calculated in the frame of density functional theory. The carbon shell of Ni nanoparticles has been found to be multilayer graphene with a significant (about 6%) amount of Stone-Wales defects. Results of the experiments evidence protection of the metallic nanoparticles from environmental degradation by providing a barrier against oxidation at least for 2 years. Exposure in air for 2 years leads to oxidation only of the carbon shell of Ni@C nanoparticles with coverage of functional groups.