Cermets are widely applied as attenuating materials due to high electromagnetic loss and better mechanical properties. In this paper, multiphase cermets composed of iron, iron oxide and alumina were successfully prepared by a two-step in situ synthesis process,which includes pressureless sintering and a selective reduction in hydrogen atmosphere. The phase composition and microstructure of cermets were analyzed by X-ray diffraction(XRD) and scanning electron microscopy(SEM), respectively. It is shown that nanosized cuboid Fe particles and octahedral Fe_3O_4 particles are distributed in alumina matrix. The permittivity and permeability of composites were tested with radio frequency impedance analyzer(0.01–1.00 GHz). The results show that permittivity presents obvious frequency dispersion. Furthermore,dielectric constants of multiphase cermets get enlarged due to the enhancement of interfacial polarization. On the other hand, there is a magnetic loss peak in permeability spectra,which indicates typical relaxation behavior. It is possible to achieve better electromagnetic attenuation property by adjusting process parameters. Cermets are widely applied as attenuating materials due to high electromagnetic loss and better mechanical properties. In this paper, multiphase cermets composed of iron, iron oxide and alumina were successfully prepared by a two-step in situ synthesis process, which includes pressureless sintering and a selective reduction in hydrogen atmosphere. The phase composition and microstructure of cermets were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. It is shown that nanosized cuboid Fe particles and octahedral Fe_3O_4 particles are distributed in alumina matrix . The permittivity and permeability of composites were tested with radio frequency impedance analyzer (0.01-1.00 GHz). The results show that permittivity presents obvious frequency dispersion. On the dielectric constants of multiphase cermets get enlarged due to the enhancement of interfacial polarization. other hand, there is a magnetic loss peak in permeability spectra, which indicates typical relaxation behavior. It is possible to achieve electromagnetic attenuation property by adjusting process parameters.