Ligand-free naked and oleylamine-capped nickel nanoparticles (Ni NPs) were size-selectively prepared in the size ranges of 5-17 nm (naked) and 13-19 nm (capped) in diameter through gas-and liquid-phase syntheses, respectively. As for the magnetic properties of both the naked and oleylamine-capped Ni NPs, the blocking temperatures, TB, increase with the size, where TB for the naked Ni NPs are significantly higher than that for the capped Ni NPs. The results indicate that the ferromagnetic core region of Ni NPs is reduced by the formation of nonmagnetic layers at the surface of the capped NPs, where their thickness is estimated to be 1-2 nm. (C) 2009 Elsevier B. V. All rights reserved.