Magnetic nanoparticles offer some attractive possibilities in biomedicine because it has the special physical properties. Magnetic hyperthermia using magnetic nanoparticles requires that the magnetic particles have not only high heating ability in low magnetic field but also to be non-toxic for biomedical use. Although the reports of bulk magnetic materials or Fe3O 4 nanoparticles have known for hyperthermia, it has not reported about application to hyperthermia and the biological responses of CoFe 2O4 nanoparticles. Therefore, we investigated the in vitro & in vivo toxicity as well as heating ability under magnetic field for application Of CoFe2O4 nanoparticles to hyperthermia.