Well crystallized nickel nanoparticles with various diameters were prepared by modified polyol process in the presence of sodium hypophosphite as reducing agent. Dense nano-structured (DNS) samples of these nanopowders were obtained using the spark plasma sintering technique. Magnetic characteristics of the powders and the dense samples were studied through the zero-field-cooling (ZFC)/field-cooling (FC) and the hysteresis measurements. Irreversibility in the (ZFC/FC) curves was observed for all samples and could be interpreted in terms of the unblocking of magnetization for the nanoparticles with diameter below 50 nm and domain wall depinning for the nanoparticles with diameter above 50 nm and for the DNS samples. Compared with bulk nickel, the nanoparticles and the DNS samples exhibited an enhanced coercivity and a decreased saturation magnetization; the relatively elevated value of the coercivity especially for the Ni nanoparticles could be due to their anisotropic morphology and their small size. The low values of the saturation magnetization could result from the inevitable surface oxidation of the powder. (C) 2010 Elsevier B.V. All rights reserved.