Abstract: The ferruginous sphere is one of the main microscopy particle components in coal fired fly ash. The microstructure and mineral characteristics of the ferruginous spheres in fly ashes from 3 coal fired power plants were studied. The ferruginous spheres in fly ash samples were investigated using scanning electronic microscopy, ICP-AES and Mössbauer spectroscopy, etc. The results are expected to reveal the physicochemical characteristics of the ferruginous sphere and provide the scientific basis for prevention of slag on the boiler heating surface and resource utilization of coal fly ash. Three types of micro-particles, namely cenospheres, plerospheres and solid spheres were identified. Hematite and magnetite occur on the surface of these spheres in the form of dendritic or octahedral euhedral crystals. The element compositions of ferruginous spheres collected from Xiao Longtan power plant include mainly Fe, Si, Al and Ca, etc.; however, those from Jiaozuo and Yaomeng power plants include mainly Fe, Si and Al, etc.. The reason was analyzed in this article. The type of mineral-phase of ferruginous spheres are divided into Fe3O4, α-Fe2O3, γ-Fe2O3 and Fe3+-silicate, and their contents are quite different among three ash samples, reflecting the difference in combustion temperature of the feed coals in boilers. With increase of the temperature, Fe3O4 is generally oxidized to γ-Fe2O3 or the co-melted solid including γ-Fe2O3 and Fe3O4 firstly, then turned to α-Fe2O3; but when exceeding 1 400 ℃, Fe2O3 turns to Fe3O4 again.