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 800℃ in Kwangryeongli clay and from 600℃ in Ildo district clay, respectively. The structural Fe2+ was completely oxidized into Fe3- at 400℃ for Kwangryeongli clay and 500℃~600℃ for Ildo district clay, respectively. The structural Fe2+ was completely oxidized into Fe3- at 400℃. For the temperature ranging from 400℃ to 700℃~800℃, two fired clays exhibit the dehydroxylation of the clay mineral. A disintegration of the clay mineral structure is observed from 700℃~800℃ to 1100℃, 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 1100℃ and the relative absorption area decreases, which might be related to the recrystallization of alluminosilicate matrix.