Cobalt nanoplatelets with hexagonal close-packed crystal structure have been synthesized through the reduction of cobalt chloride by NaH2PO2 in a mild aqueous solution without surfactant used. X-ray diffraction analysis reveals that the as-prepared samples are pure HCP cobalt. Transmission electron microscope observations indicate that the synthesized particles are plate-like in form and have the diameter between 150 and 220 nm and thickness of about 10 run. A selected-area electron diffraction analysis shows that the perpendicular direction of the Co nanoplatelets is a [001] crystallographic direction. X-ray photoelectron spectroscopy analysis implies that the surface of the Co nanoparticles was oxidized in air and formed CoO layer. Magnetic measurements reveal that the Co nanoplatelet powder samples are ferromagnetic. The saturation magnetization of the Co nanoplatelet powder samples is lower than the corresponding bulk value. The coercivity is as high as 263 Oe. (C) 2009 Elsevier B.V. All rights reserved.