The thermal behavior of ash components in two bituminous coal samples [Upper Freeport (the “UF”) and Illinois #6 (the “IL”)] was investigated under air and argon atmospheres in the temperature range of 800–1200 °C using thermal gravimetric–differential thermal analysis, X-ray diffraction transmission electron microscopy (TEM), and TEM–energy dispersive X-ray spectrometry measurements. The UF treated under air formed the needle-like crystals which were assumed to be mullite-related substances formed by transformation of andalusite, because the crystals are mainly composed of SiO2 and Al2O3. In contrast to the UF, the IL did not generate such crystals; however, when the IL was treated under air after carbonization under Ar, crystals appeared. The composition of the UF with an Al/Si ratio higher than that of the IL favored the formation of mullite-related substances, while the presence of lime in the IL inhibited the formation of mullite-related substances. Oldhamite was formed by the reaction of lime with sulfur at the carbonization of the IL under Ar and remained even at the successive air treatment. As lime was consumed, the formation of mullite-related substances was ceased to be inhibited under air after Ar treatment.