Abstract: The Fe₃O₄/polyaniline composite nanoparticles with diameters of 30~50nm were prepared. The Fe₃O₄ nanoparticles with diameters of 10~30nm were encapsulated with polyaniline (PANI) by in-situ polymerization. The inductive heating property of Fe₃O₄ nanoparticles and Fe₃O₄/polyaniline composite nanoparticles in an alternating current magnetic field was investigated. The potential of the two kinds of nanoparticles was evaluated for the localized hyperthermia treatment of cancers.The saturation magnetization M_s and coercivity H_c of Fe₃O₄ nanoparticles are 50.05emu/g and 137 Oe respectively, the Fe₃O₄/polyaniline composite nanoparticles, 26.34emu/g and 0 Oe. Exposed in the alternating current magnetic field for 30 min, the temperatures of physiological salt water suspension containing Fe₃O₄ nanoparticles or Fe₃O₄/polyaniline composite nanoparticles are 63.6℃ and 52.4℃ respectively. These two kinds of nanoparticles would be useful as good thermoseeds for the localized hyperthermia treatment of cancers.