A ferromagnetic gamma-Fe2O3-MWCNT composite was prepared by a reverse microemulsion technique, and was transformed to a Fe/Fe3C-MWCNT composite by further heat-treatment in H-2 atmosphere at 950 degrees C. TEM result suggests that MWCNTs are both surface decorated and filled with gamma-Fe2O3 or Fe/Fe3C nanoparticles. The saturation magnetization and anisotropy field are particularly strong for Fe/Fe3C-MWCNT. The reflection loss of Fe/Fe3C-MWCNT is better than that of gamma-Fe2O3-MWCNTat all frequencies between 2 and 18 GHz, resulting from enhanced magnetic loss and better matched characteristic impedance, rather than electric loss, as shown by the complex relative permeability mu(r), = mu(r)' - mu(r)'' and permittivity epsilon(r) = epsilon(r)' - j epsilon(r)''. The microwave absorbing properties can be modulated simply by manipulating the thickness of the prepared Fe/Fe3C-MWCNT composite for application in different frequency bands. (C) 2008 Elsevier B.V. All rights reserved.