Ni-Mo-B amorphous ultra-fine particles were prepared by a chemical reduction method using KBH₄ as a reducing agent. The concentration of molybdenum contained in the ultra-fine particles is about 10 at% when 30∼40 mol% of metal ions in the solution are molybdenum. The molybdenum concentration in the particles becomes the maximum value of about 30 at% in the aqueous solution where 90 mol% of metal ions are molybdenum. The structure is amorphous for all samples and the particles diameter is 20∼50 nm. The XPS Mo3d spectrum consists of Mo⁰, Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺ as-precipitated sample. The simultaneous determination of the composition of the surface film and underlying alloy indicates the enrichment of molybdenum in the surface oxide film. Nickel and boron are also composed of the metallic and oxidized states, the latter of which is associated with the surface oxide film. The XPS analysis clearly reveals that molybdenum can be reduced to the metallic state by reductive co-precipitation with nickel and boron.