Abstract: The surface morphology, microstructure, mineral components and structural porosity of the heat treated coal ash at 1300℃and 1400℃ under reducing atmosphere in a tube furnace and the slag from an entrained-flow gasifier with membrane water wall were investigated by using XRD, SEM and EDS. It is found that the surface morphology, microstructure and compositions of the heat treated coal ash are affected remarkably by the treatment temperature and the ash fluid temperature. When the treatment temperature is higher than the ash fluid temperature about 10℃, a lot of crystalline particles are formed in the heat treated coal ash, the surface morphology becomes coarse, and the microstructure is uneven. When the treatment temperature is over the fluid temperature about 100℃, the majority of crystalline particles in the heat treated coal ash are transformed to glassy amorphous phase that is distributed uniformly in the ash. The results indicate that in a simulated reducing atmosphere of the gasifier to investigate the effect of operation temperature on the structure of slag in a tube furnace is feasible.