L1 0 FePt nanoparticles with perpendicular magnetic anisotropy were fabricated on a heated MgO substrate by using an atomic deposition technique. The microstructure of the FePt nanoparticles was studied by transmission electron microscopy and high resolution transmission electron microcopy. The as-made L1 0 FePt nanoparticles are isolated and have a faceted morphology with a bimodal distribution of particle size as small as 2.5 nm. A semicoherent atomic interface between the FePt nanoparticles and the MgO substrate is observed. The room temperature coercivity of these FePt nanoparticles was measured via both superconducting quantum interference device and magneto-optical Kerr effect techniques and found to be as high as 6.7 kOe. © 2004 American Institute of Physics.