Abstract: The microwave absorbing property of nano Si/C/N composite powder has been studied in the frequency range of 8.2 GHz~18 GHz. The nano Si/C/N composite powder with its size in the range of 20 nm~30 nm was synthesized from hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃) by a laser-induced gas-phase reaction. The dissipation factors of the nano Si/C/N composite powder were high at the microwave frequencies. The microwave permittivity of the mixtures of nano Si/C/N composite powder and paraffin wax or epoxy resin can be tailored by the content of this composite powder. The unusual ε^',ε^″ and dissipation factor tgδ(ε^″/ε^') of the nano Si/C/N composite powder suspended in different matrixes were attributed to the interface effects between the nano Si/C/N composite powder and matrix. The ε^' and ε^″ of the nano Si/C/N composite powder decreased with frequency in the frequency range of 8.2 GHz~18 GHz. The difference being that the microwave resonance was not sharply peaked but rather smeared out over a large frequency range. The promising features of nano Si/C/N composite powder would be due to the more complicated Si, C and N atomic chemical environment than in a mixture of pure SiC and Si₃N₄ phase. So charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. The high ε^″ and tgδ 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 microwave absorbing materials. The microwave absorbing coating structure was designed on the basis of measured ε^' and ε^″ of the composites composed of nano Si/C/N powders and paraffin wax. Several three-layer microwave absorbing coatings with the reflection coefficient less than -8dB in the frequency range of 8 GHz~18 GHz were designed with depth of three millimeters.