The microwave absorption capability of an eastern high volatile bituminous coal from West Virginia was investigated by measuring the dielectric properties from room temperature to similar to 900 degrees C in ultrahigh purity (UHP) argon, at both 915 and 2450 MHz. The dielectric properties remain relatively constant below 500 degrees C. As the temperature continues to increase, however, the relative dielectric constant and loss factor increase dramatically. This is due to the release of volatiles, resulting in the increased electrical conductivity and higher microwave loss. The calculation of microwave penetration depth shows that the pyrolysis process significantly improves the microwave absorption capability of the coal at high temperatures. The calculated reflection losses of the coal sample suggest that the maximum microwave absorption with the reflection losses of -41.25 dB and -32.54 dB can be obtained for the coal having thicknesses of 0.14 and 0.20 m at 915 and 2450 MHz, respectively. The sample dimension has a significant effect on the overall performance of microwave absorption of coal during pyrolysis and thus on the efficiency of microwave coal pyrolysis.