The hydrophilic hollow nano-spheres and hemispheres of Co are synthesized via ethylene glycol reduction of cobalt acetate in the presence of PVP and Pd nano-particle seeds. The dimensions of the hollow core can be tuned from 100 to 300 nm by controlling the amount of Pd nano-particle seeds. The morphology of the hollow materials strongly depends on the molar ratio of the amide unit in PVP over Co and the M(w) of PVP. The hollow structure is formed when the ratio falls in the range 1-1.5 and the M(w) is over 40,000. Based on the experimental data, a possible formation mechanism of Co hollow spheres is proposed.