This study investigates the microwave dielectric properties of switchgrass at 915 and 2450 MHz under nitrogen (N-2) environment, during the process of heating it to similar to 700 degrees C. The heating process can be divided into three distinct stages, namely drying (from room temperature to similar to 200 degrees C), pyrolysis (from similar to 200 degrees C to similar to 450 degrees C), and char region (from similar to 450 degrees C to similar to 700 degrees C). The dielectric properties decreased during the drying and pyrolysis stage, but increased dramatically in the char region, indicating the strong microwave absorption capability of the char. The maximum microwave penetration depth occurred at similar to 392 degrees C, just at the high end of the pyrolysis zone. Dry biomass is a low loss material, but the microwave absorption improved significantly after the pyrolysis stage. The resulting char showed high microwave absorbing properties which can be used in many microwave absorbing applications. The experimental data were fitted using regression fit and based on this the dielectric properties model related to the temperature was developed. (C) 2014 Elsevier Ltd. All rights reserved.