Coal combustion process discharges a large amount of ash, compared with petroleum and natural gas combustions. Coal ash particles melt and partially volatilizes at high temperature zones, causing many problems. In this study, ash deposition experiment was carried out, using simulated heat transfer tubes made of thermal spraying material. Coal ash was injected into the furnace via high penetrance at a high temperature. The molten slag formation ratio of their chemical composition in a single coal was estimated by the thermodynamic equilibrium calculation and the change of ash deposition characteristics was quantitatively discussed.