Abstract: 3 ash samples with nearly same chemical compositions were obtained by blending two raw coal ashes and mixing with different additives. There exists obvious difference among the ash fusion temperatures (AFTs) of the three ash samples. Mineral transformation at high temperature (>800 ℃) was studied by SEM-EDX and XRD. The results show that different regulative methods have different impact on AFTs. This is because that the mineral composition at high temperature is related with the type of chemical elements and their occurrence in minerals. Both of them determine the ash fusion characteristics.