This paper describes a new approach, based on the Mie theory, to measure the dielectric properties of lossless materials at temperatures greater than 1500 degrees C. For the reliable operation of microwave transmitting materials in harsh environments, it is crucial to correctly characterize the permittivity under various temperature conditions. Heating and measurement systems using a propane torch and a single horn antenna were designed to estimate such dielectric properties. The reflection spectrum of Al2O3 ceramics at room temperature was determined using the Mie theory to derive the permittivity and validate the approach. High-temperature dielectric constants are derived from simulated values, which are reliable and have only a slight slope as a function of temperature. These results indicate that the permittivity measurement technique can provide powerful information for the optimal design and accurate evaluation of the dielectric properties of various lossless materials at high temperatures.