The microwave permittivity of nano Si/C/N composite powders suspended in paraffin wax has been studied at the frequency range of 8.2—18GHz. The nano Si/C/N composite powders were synthesized from hexamethyldisilazane ((Me3Si)2NH) (Me:CH3) by a laser induced gas phase reaction. The dissipation factors of the nano Si/C/N composite powders are high at the microwave frequencies. The microwave permittivity of the mixture of nano Si/C/N composite powders and paraffin wax (or other dielectric materials) can be tailored by the content of the composite powders. And ε′, ε″ and tan δ increase with the volume filling factor ( v ) of nano Si/C/N composite powders. The ε′ and ε″ can be effectively modeled using second order polynomials ( ε′, ε″=Av2+Bv+C ). The ε′ and ε″ of the nano Si/C/N composite powders decrease with frequency at the frequency range of 8.2—18GHz. The difference being the microwave resonance is not sharply peaked but rather smeared out over a large frequency range. The promising features of nano Si/C/N composite powders would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si3N4 phase. The SiC microcrystallines in the nano Si/C/N composite powders dissolve a great deal of nitrogen. The local structure around Si atoms changes by introducing N into SiC. Carbon atoms around Si are substituted by N atoms. So there exist a large number of charged defects and dangling bonds in the nano Si/C/N composite powders. Thus charged defects and quasi free electrons move in response to the electric field, diffusion or polarization current resulting from the field propagation. The high ε″ and dissipation factor tan δ(ε″/ε′) of Si/C/N composite powders are due to the dielectric relaxation.