Abstract: The particle size distribution of ultrafine powder was treated by three grades according to some present view points so as to clear up the confusions of misunderstanding definitions and concepts. The special properties of ultrafine metallic particles which are different from usual metal block or powder can be illustrated by detail description of their surface and volume efficiency(including quantum size efficiency) and of the magic numbers structure of atomic cluster. The new achievements in applications of ultrafine metallic particles for astronavigations atomic energy, military, electronics, chemistry, metallurgy and other industry fields, and also in the aspects of medical and biological engineering are emphatically outlined.