Based on the X-ray powder diffraction (XRD) and M ssbauer spectroscopy, the thermal behavior and phase transformations of two clays are investigated for raw and fired conditions, which are collected from Kwangryeongli and Ildo district in Cheju Island. M ssbauer spectra at room temperature and 20for two clays show that paramagnetic Fe3- is the structural iron of the layer silicate and ferrihydrite, and superparamagnetic goethite has about 50% of total iron contents. The XRD peaks of hematite for the fired clays appear from 80$0^{circ}C$ in Kwangryeongli clay and from $600^{circ}C$ in Ildo district clay, respectively. The structural Fe2+ was completely oxidized into Fe3- at 40$0^{circ}C$ for Kwangryeongli clay and 50$0^{circ}C$~$600^{circ}C$ for Ildo district clay, respectively. The structural Fe2+ was completely oxidized into Fe3- at 40$0^{circ}C$. For the temperature ranging from 40$0^{circ}C$ to $700^{circ}C$~80$0^{circ}C$, two fired clays exhibit the dehydroxylation of the clay mineral. A disintegration of the clay mineral structure is observed from $700^{circ}C$~80$0^{circ}C$ to 110$0^{circ}C$, followed by the onset and spread of vitrification process. It is also shown that well-crystallized hematite phase is formed at the temperature higher than 110$0^{circ}C$ and the relative absorption area decreases, which might be related to the recrystallization of alluminosilicate matrix.