Nickel fine particles with diameters under 600 nm were synthesized rapidly and continuously in water at temperatures ranging from 300 to 380degreesC and at 30 MPa. Nickel particles formed through the following two steps: formation of Fe3O4 nuclei by hydrothermal synthesis from FeSO4 aqueous solution and precipitation of nickel on the surfaces of the nuclei (i.e., Ni coating) by hydrogen reduction from Ni(CH3COO)(2) aqueous solution. The substance 1,10-phenanthroline was used as a complex for preventing hydrolysis of Ni2+ because hydrolysis promotes formation of nickel oxide particles at higher temperatures. Thickness of Ni shell on the Fe3O4 nuclei increased with increasing reaction temperature and Ni/Fe molar ratio in the feed solution. The 1,10-phenanthroline was thermally stable up to 380degreesC for residence times of ca. 19 s, which means that recycle after product formation is probably possible.