FePt nanodot with L10 face-centered-tetragonal (fct) structure has attracted considerable attention owing to its potentials for new applications including a high-density magnetic data storage and a novel nonvolatile memory which are produced by its high magnetocrystalline anisotropy1. FePt nanodots dispersed in a SiO2 film (FePt nanodot film) were formed by a self-assembled nanodots deposition (SAND) method in which FePt and SiO2 are co-sputtered in a high-vacuum RF magnetron sputtering equipment. Furthermore, FePt nanodot film with a monolayer of FePt nanodots was formed by annealing at 800°C for 1 h, due to the agglomeration of FePt atoms diffused in SiO2 films. The electron holography was a powerful tool to study the single-domain FePt nanodots. In this study, not only the phenomenon of FePt diffusion was evaluated, but also the ferromagnetic phase of monolayer FePt nanodots was analysed by electron holography10-11.