A method for measuring the complex permittivity for absorbing materials has been developed for temperature up to 1000° C. The test apparatus consists of an open ended flanged circular waveguide propagating TE11mode, which radiates into material under test placed in front of the flange. Electromagnetic analysis by modal method permits the calculation of the field in different zones of the structure. Complex permittivity is deduced by both theoretical and experimental reflection coefficient values. A solar source is used to realize the complex permittivity measurement as a function of temperature. Microwave measurements were made with different samples for different temperature laws. Reproducibility of thermal tests permits the validation of the method.