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Multiple interface polarization along with conductive loss, dipolar polarization and magnetic loss are contributed to excellent electromagnetic wave absorption performance. • NiCo alloy nanospheres and CoNiO 2 nanoflowers were successfully embedded on the porous carbon skeleton. • The outstanding reflection loss was up to −74.3 dB in 3.8 mm and widest effective absorption was 6.32 GHz in 2.4 mm. • The synergistic effect of magnetic loss and dielectric loss are contributed to excellent absorbing performance. A series of NiCo/CoNiO 2 @C hybrid composites were successfully prepared by a hydrothermal method and subsequent heat-treatment process. Porous carbon was synthesized through a fabric carbonization process derived from fish skin. The micro-morphology and minor component of NiCo/CoNiO 2 @C hybrid composites could be tuned by controlling the adjunction amount of Co2+ and Ni2+. The NiCo/CoNiO 2 @C hybrid composite exhibited strong electromagnetic wave absorption performance when the adjunction amount of Co2+ and Ni2+ was 0.4 mmol and 0.2 mmol. The optimal reflection loss could up to −74.3 dB at the matching thickness of 3.8 mm, while the corresponding widest effective absorption bandwidth (reflection loss values lower than −10 dB) is up to 6.32 GHz covering from 11.78 GHz to 18.0 GHz at the matching thickness of 2.4 mm. Based on the Maxwell-Garnet theory, the pore size of porous carbon materials could influence the dielectric constant which has a great effect on impedance. Previous work has illustrated that porous carbon carbonized at 650 °C processes the proper pore size for excellent impedance matching. Besides, NiCo alloy nanosphere and CoNiO 2 nanoflower would provide magnetic loss and interface polarization for attenuating electromagnetic wave energy. Moreover, the conductive loss derived from porous carbon and dipolar loss which originated from the defects are also beneficial to decay electromagnetic energy. This work indicates that the as-prepared NiCo/CoNiO 2 @C hybrid composites accompanied with excellent electromagnetic wave absorption performance could act as a promising absorber to deal with the increasingly serious electromagnetic pollution. [ABSTRACT FROM AUTHOR]