An analysis of the chemical form of mercury in the by-products produced from a coal-fired power plant was performed by a combining temperature programmed desorption (TPD) method and cold vapor atomic absorption. The TPD curve of the model sample for each mercury compound showed a unique peak temperature, and the chemical form of mercury was identified by comparing the peak temperature with that of the by-product sample. A linear combination of the TPD curve of each model sample was curve-fitted to that of the by-product sample. The ratio of each form of mercury present in the by-products was calculated from the area ratio of the TPD curve of the model sample used in the analysis. The mercury in the fly ash used in this study mainly existed in a form bound to sulfur, and the mercury that adsorbed on carbon was not found despite the very high unburned carbon content. In contrast, the study found that the mercury adsorbed to carbon was the main form of mercury in desulfurization gypsum. These findings suggested that mercury-adsorbed carbon was formed when the mercury that transitioned into the liquid phase in a desulfurization equipment was adsorbed onto the unburned carbon.