Experimental research was conducted on the relationship between the unburned carbon content in fly ash and the microwave power attenuation at frequencies ranging from 1.0 to 4.0 GHz. The experimental results show that the unburned carbon in fly ash exhibits the frequency characteristics of microwave power attenuation. At some frequencies, the microwave power attenuation increases with the carbon content. At other frequencies, the relationship between them changes. There is also a range of frequencies where the loss decreases while the unburned carbon content increases. A detailed analysis on the mechanisms of this phenomenon is made based on the conductivity loss, absorption and dispersion theories. (C) 2010 Elsevier Ltd. All rights reserved.