The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me 3Si) 2NH) (Me:CH 3) and SiH 4–C 2H 2, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part ( ε′) and imaginary part ( ε″) of the complex permittivity, and dissipation factor ( tg δ=ε″/ε′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si 3N 4, SiO 2, and Si, especially the tg δ . The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si 3N 4 phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp 3 dangling bonds (of silicon and carbon) and unsaturated sp 2 carbons. The high ε″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.