The study was undertaken to investigate the changes of mineral matters in coal during heat treatment and its effect on structure and dielectric properties of generated chars. An acid treatment method was adopted to remove mineral matters in coal for preparing chars at 850-1600 degrees C using an electric furnace. Dielectric properties of coal chars were determined with the transmission/reflection method. Also, phase structure of coal chars was examined by X-ray diffraction (XRD). Significant transformation was found in the presence of mineral matters during heat treatment. Especially the process of creating silicon carbide (SiC) above 1300 degrees C in carbothermal reduction played a catalytic role on char structure. Consequently, dielectric properties were improved with the increase of structural ordering. These results could be instructive for the application of mineral matters in microwave technique at higher temperatures.