The magnetic properties and the synthesis behavior of self-assembled Co nanorods and nanoballs were analyzed. The Co nanoparticles were synthesized from octacarbonyldicobalt in o-dichlorobenzene solution in oleic acid, using rapid pyrolysis. It was observed that the shape, the size and the size distribution of the nanoparticles were influenced by the preparation conditions. The size, structure, and morphology of the Co nanoparticles were investigated using transmission electron microscopy images, selected area electron diffraction patterns, and energy-dispersive x-ray spectra. The as-synthesized rod-shaped Co nanoparticles were found to exhibit paramagnetic behavior at 293 K, and feromagnetic behavior at 100 K, under 20000 Oe.