Synthesis of Ni nanoparticles having an average diameter of 8.1 nm by reduction of NiO nanoparticles was carried out at 673 K. HCOOH was used as a reductant, which decomposes to CO and/or H-2 at higher temperatures. NiO nanoparticles having an average diameter of 3.8 nm were obtained from Ni(NO3)(2) + KOH aqueous solution by a flow-through supercritical water method at 673 K and 30 MPa. Effect of HCOOH concentration on particle size and saturation magnetization was investigated.