A method to produce dispersion of magnetic metal particle aggregates is provided. FePt nanoparticles are uniformly dispersed with the particles separated at controlled inter-particle spacing. The magnetic metal particle aggregate is composed of magnetic metal particles whose main components and the contents thereof are represented by the following general formula (1): [T.sub.XM.sub.1-X].sub.YZ.sub.1-Y, (1) where T is one or both of Fe and Co, M is one or both of Pt and Pd, Z is at least one member selected from the group composed of Ag, Cu, Bi, Sb, Pb and Sn, X represents 0.3.about.0.7, and Y represents 0.7.about.1.0, the balance being impurities unavoidably incorporated during production, wherein the proportion of face-centered tetragonal structure is in the range of 10.about.100%, the particles have an average diameter of 30 nm or less, and the particles are in a dispersed state spaced apart from one another.