The invention provides a powder of fine particles of an alloy of platinum-family metal having a surface with a large amount of unevenness that is suitable as a catalyst metal. It is a powder of an alloy containing T and M in a composition ratio represented by the formula [T.sub.XM.sub.1-X], where T is one or two or more of the elements Fe, Co and Ni, M is one or two or more of the elements Pt, Pd and Ru, X represents a numerical value in the range 0.1-0.9, and wherein: its average grain size as measured under TEM observation (D.sub.TEM) is 50 nm or less, and under TEM observation, a plurality of protruding points is observed on the surface of the particles and indentations are observed between the protruding points, making it a fine powder of alloy particles with an uneven surface. This powder of fine alloy particles has a crystal structure with a face-centered cubic (fcc) structure, and its single crystallinity (D.sub.TEM)/(D.sub.X) is 1.50 or greater. In addition, the average grain size of the alloy particles as found by the dynamic light-scattering method is 50 nm or less.