Abstract: Fly ashes, obtained from three commercial CFB boilers with burning Fujian anthracites, were studied in this paper. By using standard dry-sieving techniques and LOI method, the size distributions of fly ashes and the carbon contents of different size fractions were measured. With the help of SEM and EDX technique, the particle superficial property of fly ash was examined and divided into carbon (soot, or graphite), char, and slag. The residual carbon was enriched from fly ash by the floating-separation technique, and its reactivity was then measured via a non-isothermal TGA experiment. The results was compared with that of its corresponding parent coal. Furthermore, in a laboratory pilot scale fluidized bed furnace, the source of the unburned carbon in the fly ash was explored. It is shown that for CFB boilers using a single cyclone separator, the largest amount of fly ash particles lies in the particle size range from 0.0385mm to 0.0500mm, and so does the unburned carbon fraction; and for CFB boiler equipped with two separators, the largest amount of fly ash particles and unburned carbon fraction lies in a particle size range of 0.0500mm~0.0750mm. There are three kinds of particles existing in fly ashes, which are granular unburned carbon, flocculent slag and char. The reactivity of the unburned carbon in fly ashes is relatively higher than that of its parent feeding coal. It can be induced from this study that the unburned carbon in fly ashes comes primarily from the fines existed in the fed coal and produced by the attrition and fragmentation of coal particles, which cannot be totally burned in the earlier stage of combustion. The clarain, which is of high reactivity and frangible, may be the main originations of unburned carbon in fly ash.