Dielectric properties of quartz glass and $Si_3N_4$ are investigated using the waveguide method from room temperature to $800^{circ}C$. For the case of dielectric constant, $Si_3N_4$ showed similar increase with quartz glass up to $300^{circ}C$, but less increase from $300^{circ}C$ to $800^{circ}C$. For the case of loss tangent, those showed gradual increase with temperature except of some temperature points. The loss tangent of $Si_3N_4$ and quartz glass increased up to 18.2% and 12.5% respectively. Through these researches, high temperature dielectric properties of silicon nitride materials are characterized.