Magnetite nanoparticles decorated CNTs/PANI multiphase heterostructures were prepared by polymerization of aniline monomer and an additional process of the coprecipitation of Fe2+ and Fe3+. Scanning electron microscopy and transmission electron microscopy observation indicated that the monodispersed magnetite nanoparticles were uniformly decorated on the surface of CNTs/PANI. The formation of magnetite nanoparticles on CNTs/PANI was mainly through a preferentially position-selective precipitation process. More interestingly, a portion of Fe3O4 nanoparticles was found to form core-shell structures with PANI. The effects of different additional amounts of NH2Fe(SO4)(2)center dot 6H(2)O reactant on the magnetic properties and microwave absorbing performances of CNTs/PANI/Fe3O4 heterostructures were investigated. The CNTs/PANI/Fe3O4 multiphase heterostructures were proved to be superparamagnetic. The microwave absorption measurement showed that the CNTs/PANI/Fe3O4 samples under 1.5 g of NH2Fe(SO4)(2)center dot 6H(2)O condition exhibited much more effective absorption performance. These results suggested the novel CNTs/PANI/Fe3O4 multiphase heterostructures with PANI as the second phase may be potential candidate for microwave absorption systems.