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.