This study aims at exploring an effective route in the in situ graft polymerization of aniline from Fe3O4 nanoparticles. To this goal, Fe3O4 magnetic nanoparticles were prepared by coprecipitation method using ammonia solution as the precipitating agent, and were characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thereafter, polyaniline  (PANI) grafted magnetite nanoparticles were successfully synthesized by the in situ chemical oxidative polymerization of aniline monomer by ammonium peroxodisulfate (APS) from the surfaces of the aminopropyl magnetite nanoparticles with a dispersion polymerization method. The chemical grafting of polyaniline from magnetite nanoparticles were confimed by using FT-IR, ultraviolet-visible (UV-Vis) spectroscopy and thermogravimetric analysis (TGA), and also the dispersion state of Fe3O4 nanoparticles in the polyaniline matrix was examined by TEM. In comparison to the pure polyaniline, the polyaniline/Fe3O4 nanocomposite shows to have higher decomposition temperature.