Ni‐P ultrafine amorphous alloy particles have been prepared by reacting nickel chloride and sodium hypophosphite in an aqueous solution. X‐ray diffraction revealed the amorphous structure of the particles and transmission electron microscopy showed that the particles are spherical, with an average diameter of about 150 nm. X‐ray photoelectron spectroscopy confirmed that all nickel and phosphorus atoms are in the elementary state on the surface of the particles, which implied the alloying state of the particles. The dynamic process of the reaction has been studied. It is found that the reaction ended within 65 min at apH value of 10 and 351 K when the sodium hypophosphite was exhausted, and the unreacted solid residue Ni(OH)2should be washed out with ammonia solution to obtain ‘‘pure’’ Ni‐P ultrafine amorphous alloy particles.