The magnetite (Fe3O4) nanoparticles were prepared by coprecipitation of Fe3+ and Fe2+ with aqueous NaOH solution. The Fe3O4/polyaniline (PANI) magnetic composite nanoparticles with core-shell structure with diameter of 30-50 nm were prepared via an in-situ polymerization of aniline in aqueous solution containing Fe3O4 magnetic fluid. The inductive heat property of Fe3O4/polyaniline composite nanoparticles in an alternating current (AC) magnetic field was investigated. The potential of Fe3O4/polyaniline nanoparticles was evaluated for localized hyperthermia treatment of cancers. The saturation magnetization Ms and coercivity Hc 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 (AC) magnetic field for 29 min, the temperatures of physiological saline suspension containing Fe3O4 nanoparticles or Fe3O4/polyaniline composite nanoparticles are 63.6 °C and 52.4 °C respectively. The Fe3O4/polyaniline composite nanoparticles would be useful as good thermoseeds for localized hyperthermia treatment of cancers.