Nickel particles were directly synthesized from 0.1 mol/kg nickel formate aqueous solution in supercritical water at 673 K and 30 MPa for 3-30 min with a titanium alloy autoclave. This method uses a specific property in which supercritical water forms a homogeneous mixture with hydrogen, which is produced by thermal decomposition of formate anion. In this environment, Ni2+ is easily reduced to Ni and Ni crystals precipitate. High-crystallinity Ni particles in sub-micrometer scale were obtained, and the hysteresis curve of the ferromagnetic crystals could be seen.