Experiment were conducted on the selected ashes for understanding the effects on slag flow properties including slag viscosity and temperature of critical viscosity. XRD applied to determine the component of ash. The coaxial cylinder sensor equipment was employed for viscosity measurement. All composed of high density alumina are placed in a high temperature furnace. For measuring, the furnace was heated to 1550℃. At 1550℃, a few grams of coal ashes were fed from top. After feeding, it took enough to allow the ash to completely melt and degas. Then, it was to decrease temperature from 1550℃ to 1100℃, the cooling rate was 2℃/min. It was obtained torque as a function of temperature during decreasing temperature. The results was converted to viscosity for quantifying by mathematical method and analyzed with components of ash. It was confirmed that SiO<SUB>2</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB> as refractory contents actin component affected to decrease viscosity whereas CaO,FeO increased viscosity.