The present invention discloses a magnetic nanocomposite for inhibiting/treating cancer and a method for fabricating the same. The magnetic nanocomposite comprises a core formed of a plurality of magnetic nanoparticles made of ferric ferrous oxide (Fe3O4); a shell made of a carboxy-functionalized polyaniline; and an anti-tumor medicine bound to the external surface of the shell. The method of the present invention fast fabricates the magnetic nanocomposite in a simple way. The medicine of the present invention has a longer half life and a better thermal stability. The present invention disperses the water-insoluble medicine in water uniformly to decrease the biological rejection. Moreover, the magnetic nanocomposite of the present invention is guided to the nidus by an external magnetic field to increase the local concentration of the medicine and provide an effective chemotherapy. Therefore, the present invention has competitive advantage over the conventional BCNU.