A convenient and efficient method of reversible phase transfer of ferromagnetic L1(0)-FePt nanoparticles between aqueous and organic phases was developed. From the SiO2-coated L1(0)-FePt nanoparticles prepared by the "SiO2-nanoreactor" method, stable aqueous dispersions of L1(0)-FePt nanoparticles were prepared first by using tetramethylammonium hydroxide (TMAOH). Replacement of TMAOH by mixtures of oleic acid and oleylamine enables transfer of the L1(0)-FePt nanoparticles from aqueous to organic phase. The L1(0)-FePt nanoparticles could be subsequently transferred to aqueous phase by replacing the hydrophobic ligands with TMAOH. Magnetic properties were unchanged during the phase transfer, suggesting that these L1(0)-FePt nanoparticles will be of great importance not only for applications in biomedical fields but also for fabrication of high-performance magnetic devices via bottom-up processes.