Abstract: SiC/C foams and their monoliths of the same compositions, with various electrical conductivities were prepared by polymer sponge replication/carbonization and hot-press solidification/carbonization respectively. Both materials have a threedimensionally connected pore structure with the former on a macroscopic scale and the latter a microscopic scale. The electromagnetic parameters of the SiC/C foams and the monoliths versus their electric conductivities were measured at a frequency of 2 450 MHz by the TE10 resonant cavity perturbation method. Results show that when the electrical conductivities of the SiC/C foams and the monoliths increase gradually, their dielectric constants ε_r′ increase gradually and their dielectric losses tg δe exhibit a maximum. The magnetic loss tg δm of the SiC/C foams increases continually while that of the monoliths first increases quickly then decreases slowly with increasing electric conductivities. When the effective electric conductivities of the SiC/C foams and the monoliths are the same, the ε_r′ values of the SiC/C foams are about a half and the tg δe values are at least 2 times larger than those of the monoliths, while the tg δm values of the monoliths are more than 4 times those of the SiC/C foams. The SiC/C foams and the monoliths are both non-magnetic, and their magnetic losses are found to be caused by their special structures interacting with the incident electromagnetic waves. Thus it is apparent that this kind of magnetic loss is extrinsic.