Abstract: Ash fusion determinator and high-temperature rotational viscometer are used to study the fusibility and viscosity-temperature characteristics of coal ash and gasifier slag from Baodian and blended coal. Under high temperature conditions, viscosity-temperature characteristics of coal ash are similar with those of slag. According to the composition of coal ash and slag and its location in Al2O3-SiO2-CaO-FeO quaternary phase diagram and the minerals change of coal ash and slag near the critical viscosity, reasons for the differences of fusibility and viscosity-temperature characteristics of coal ash and slag are analyzed. Analysis and experimental results are in good agreement.