Phase transition to L10 for the FePt crystal requires a temperature approximately 600 °C; however, in this temperature, their size control is exceptionally imperative. Two methods of particle size control with unique advantages have been combined. On the SiO2 wafers, a thin layer of FePT/carbon nanotubes (CNTs) has been coated and post-annealed at 700 °C under electron flux. Results show the Hc and Mr/Ms of FePt/CNT nanocomposites is raised to 0.75 T and 0.9, respectively, and the average size of the FePt nanoparticles (NPs) is about 5 nm.