Actual and potential applications of metallic cobalt rely on its crystal size- and phase-dependent magnetic properties. On this basis, the present work addresses the synthesis and characterization of nanocrystalline cobalt particles produced through a modified polyol-based approach, which is conducive to the formation of epsilon (ε-Co) or hcp-Co phases. Metastable -Co could be converted into hcp-Co by controlled additions of polyvinyl pirrolidone (PVP) in trimethylene-glycol. XRD evidenced the progressive conversion of the ε-Co phase into the hcp-Co phase, as the PVP/Co mole ratio increased from 0 to 11. Magnetic measurements confirmed the strong influence of synthesis conditions on crystal structure and hence, on magnetic properties; the coercivity of the products varied from 134 Oe for ε-Co to 752 Oe for the hcp-Co. These values agreed with the soft magnetic nature of epsilon and hard magnetic character of hexagonal phases of cobalt. © 2011 American Institute of Physics.