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FePt nanoparticles with a size of 3 nm and thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles were prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirmed the phase transition from the disordered face-centered cubic structure to the L10 structure with the chemical ordering parameter of 0.62±0.05. Analysis in blocking temperature and fitting of temperature dependence of switching field reveals that the transformed 3 nm nanoparticles have a magnetic anisotropy constant of (2.8±0.2) x 106 J/m3, smaller than those for the bigger particles and the fully ordered L10 bulk phase.