Effects of substitution of Pt by Cu on the structural and morphological changes of Fe-Pt nanoparticles with chemical homogeneity and mono-dispersion, which were synthesized by the reverse micelle method, have been investigated by in-situ electron diffraction and HREM observation. In the Fe-Pt particles, initially with a chemically disordered face-centered cubic phase (A1), the phase transformation from A1 to a chemically ordered face-centered tetragonal phase (L1₀) took place between 650 and 680°C. Polycrystalline Fe-Pt particles were reconstructed into A1 single crystals between 550 and 650°C, followed by phase transformation from A1 to L1₀. The coalescence began almost concurrently with the phase transformation, i.e., between 650 and 680°C. Then, they turned to round-shaped single crystalline particles between 680 and 720°C. In the Fe(Pt,Cu) particles, the phase transformation from A1 to L1₀ occurred between 550 and 600°C. Same processes, that is, the reconstruction to single crystals, the phase transformation from A1 to L1₀, the coalescence and the round-shaped particle formation were also observed in the Fe(Pt,Cu) nanoparticles. The temperatures, at which these processes took place, were substantially lower than those for the Fe-Pt nanoparticles.