A new precursor for the sonochemical preparation of amorphous nickel, Ni(cyclooctadiene)2, yielded relatively large (200 nm) amorphous nanoparticles composed of nickel and carbon atoms. Small nickel particles were dispersed all over the particle. When these particles were heated slightly above their crystallization temperature, much smaller particles (5 nm) of encapsulated crystalline nickel in amorphous carbon were obtained. The XPS spectrum reveals that the crystallization process is also accompanied by the reduction of the surface Ni2+ ions by the amorphous carbon atoms. The DSC measurements substantiate this assumption.