The process of phase transformation in individual Fe-Pt and Fe-Pt-Cu nanoparticles synthesized by the reverse micelle method with chemical homogeneity and mono-dispersion has been investigated by in-situ high-resolution electron microscopy (HREM) observation and in-situ nano-beam diffraction (NBD). The Fe-Pt particles, initially polycrystalline with the chemically disordered fcc (A1) phase, were reconstructed into A1 single crystals between 550 and 650°C, followed by a phase transformation from A1 to the chemically ordered fct (L10) phase between 650 and 680°C. The coalescence began almost concurrently with the phase transformation. They were transformed into round-shaped single-crystalline particles between 680 and 720°C. Similar processes were also observed in the Fe-Pt-Cu nanoparticles. The temperatures at which these processes occurred were substantially lower than those required for the Fe-Pt nanoparticles. We investigated the magnetic-field distribution of a submicron-size island comprising isolated L10 Fe-Pt nanoparticles magnetized along one direction by using in-situ electron holography at elevated temperatures. Although the magnetization decreased between 212 and 412°C to 25% of the strength at 25°C, it recovered 67% of the initial strength during cooling. However, when an island was heated to 512°C, the magnetization diminished and did not recover during cooling. The Curie temperature (Tc) was determined to be 350°C and was in good agreement with the Tc determined by bulk measurements, which was approximately 100°C lower than the Tc for bulk Fe55Pt45.