In the current work, we synthesized Ni/C composites from Ni/cellulose composites and analyzed how well the products absorbed microwaves. The Ni/C composite products showed a maximal reflection loss (RL) of −53.8 dB at 4.72 GHz with a sample thickness of 4.5 mm, and the RL values exceeded −10 dB covers almost the whole tested frequency ranges (2–18 GHz). These results were attributed to a combination of the magnetic loss and the dielectric loss. This work is expected to contribute to the development of materials that absorb a broad region of the electromagnetic wave and to the recycling of cellulose.