By using inorganic Fe3O4 nanoparticles as nucleation sites, Fe3O4/polyaniline magnetic nanocomposites are successfully synthesized by chemical oxidative polymerization method. The morphology and properties of Fe3O4/polyaniline nanocomposites are characterized by XRD, FTIR, TEM, DSC, TG and VSM. XRD, FTIR and DSC jointly indicate that the composites comprise Fe3O4 and polyaniline. TEM micrographs indicate that the Fe3O4 magnetic nanoparticles have an average diameter less than 20 nm and achieve better property of dispersion after composited with polyaniline. With the content of polyaniline increasing, Fe3O4/polyaniline composites change from aggregated particles to fibers. Fibrous composites achieve excellent thermal stability and attractive polyaniline content dependence of magnetism. TG analyses reveal that apparent weight loss of the fibrous composite appears at higher temperature than the granular Fe3O4/polyaniline composite does. With the increasing of the content of polyaniline, the magnetism of Fe3O4/polyaniline composite change from ferromagnetism to paramagnetism, while the magnetization decreases firstly and then increases. The formation of fibrous Fe3O4/polyaniline composite is favorable for the magnetization.