Abstract: With the residual carbon in coal hydrogasification semicoke as both reducing agent and dielectric component, Ni/carbon-based composite materials for microwave absorption were prepared. The synthesis process mainly involved loading of Ni species via an impregnation of nickel nitrate solution and then an in-situ carbothermic reduction. The effects of Ni load on the microstructure and properties as well as the related mechanism were studied. The experimental results showed that the electromagnetic parameters could be readily regulated by changing the Ni load, which occurred as a result of the accompanied changes in carbon content, graphitization degree, as well as the number of interfaces and defects. Hence, a good impedance matching could be easily achieved. At a carbothermal reduction temperature of 700 ℃, the composite with 20% Ni load showed the best microwave absorption performance. For a coating thickness of 2.5 mm, the minimum reflection loss was −42.6 dB and the corresponding effective bandwidth was 4.1 GHz; while the effective bandwidth could be up to 5.6 GHz under 2 mm coating thickness. The dominant microwave absorption mechanism was the dielectric loss, which mainly derived from the conduction loss due to graphite carbon and the polarization relaxation losses because of the existence of interfaces and defects.