Cobalt nanoparticles have been prepared by gamma-radiolysis in three different forms, namely, aqueous sol, self-supporting powder, and dispersed on the surface of Al2O3, and characterized for their particle size distribution and metallic characteristics, using a variety of techniques. Air-stable ferromagnetic cobalt particles were found to possess the face-centered cubic structure with an average crystallite size of 5-6 nm. Magnetization studies carried out on frozen cobalt sol at 5 K also showed ferromagnetic behavior. The agglomeration of these crystallites produced particles in the range of congruent to 100 nm.