FePt nanoparticles with thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles are prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirm the phase transition from the disordered face-centered cubic structure. In this paper, 3 nm FePt nanoparticles are first synthesized by superhydride reduction of Fe and Pt. Transmission Electron Microscopy (TEM) images show that the hard magnetic FePt are agglomerated after annealing at 675°C. Scanning Electron Microscopy (SEM) images indicate that the size of FePt nanoparticles increase by increasing of the annealing temperature.