Highly conductive nanocomposites were prepared by in situ polymerization of polyaniline (PANi) and multi-walled carbon nanotubes (MWCNTs) as electromagnetic interference shielding materials. gamma-Fe(2)O(3) nanoparticles were also incorporated in the nanocomposites to improve the ferromagnetic properties. SEM and TEM images showed the uniformly coated PANi on the surface of MWCNTs and gamma-Fe(2)O(3). XRD peaks also confirmed the presence of MWCNT and gamma-Fe(2)O(3) in the nanocomposites. The nanocomposites showed significant improvement in permittivity, permeability, and electromagnetic interference shielding efficiency due to the conductive effect of MWCNTs and the magnetic effect of gamma-Fe(2)O(3). The electromagnetic interference shielding efficiency of nanocomposites increased up to 34.1 dB due to the synergetic effect of reflection and absorption of electromagnetic interference by MWCNTs and gamma-Fe(2)O(3) additives.