Abstract: The silicon nitride fiber has excellent high temperature resistance and wave transmission capability, which is an ideal reinforcement using for high-temperature wave-transparent composites. In this paper, preparation and test methods for the dielectric properties of ceramic fibers tested by high-Q cavity method were studied and optimized for continuous ceramic fibers. The study shows that the fiber content in the dielectric test sample should be no less than 20wt%, and less fiber content tended to result in a relatively smaller value of the fiber dielectric constant. Meanwhile, the length of chopped fiber has effect on the repeatability of the fiber dielectric properties. The samples made by the chopped fiber with the length no more than 1.0 mm are of high quality and the resulted fiber dielectric data are more reliable. In terms of data processing, the applicability of three dielectric mixing models of Lichtenecker, Bruggeman and Looyenga was discussed in comparison. Finally, the permittivity of quartz fibers was calculated based on Lichtenecker dielectric constant logarithmic mixing rule, which was consistent with the reported data. It has been found that the silicon nitride fiber has a permittivity of 4.4 and a loss tangent of 0.0005 at 10 GHz, demonstrating excellent low dielectric. Furthermore, it shows that the surface sizing agent has a significant effect on the dielectric properties particularly the loss tangent of the silicon nitride fiber, which is associated with its surface polarity characteristics of the silicon nitride fiber.