Abstract: The present disclosure relates to a method and an apparatus for preparing nanosized metal or alloy nanoparticles by depositing metal or alloy nanoparticles with superior size uniformity on the surface of a powder as a base material by vacuum deposition and then dissolving or melting the base material using a solvent or heat. The method solves the problems of the existing expensive multi-step synthesis method based on chemical reduction and allows effective synthesis of metal or alloy nanoparticles with very uniform size and metal or alloy catalyst nanoparticles supported on carbon at low cost.