The present paper presents a facile hydrothermal method to synthesize FeNi3@C nanowires by reducing Ni(NO3)2·6H2O and Fe(NO3)2·9H2O with hydrazine hydrate in ethanol and glucose solution. The FeNi3@C nanowires have diameters ranging from 5 to 15nm and length ranging from 150 to 250nm, which are self assembled by FeNi3@C nanocapsules with onion-like carbon shells. The electromagnetic properties of FeNi3@C nanowires–paraffin composites have been investigated in detail. The Cole–Cole semicircle approach is adopted to explain the dielectric resonance, due to a cooperative consequence of the core/shell interfaces and the dielectric carbon shells. The coexistence of natural resonance and exchange resonance result in strong magnetic loss. For 2.0mm thickness layer, an optimal reflection loss (RL) of −43.3dB is observed at 9.91GHz and RL exceeding −10dB is obtained at 8.7–13.78GHz. The absorbent with a thickness of 1.2mm has a RL of −28.49dB at 18.0GHz. The FeNi3@C nanowires can be a potential material for fabricating microwave devices, due to the synergy of dielectric loss and magnetic loss and the quarter-wavelength cancellation.