Abstract: According to the contradiction between ash melting theory and practice, an uneven melting theory was proposed. A typical Jincheng coal from Shanxi, China, was selected, which was ground into the particle size similar to the industrial use. It was separated into different density fractions through float and sink tests, and its chemical composition and mineral composition were analyzed with XRF and XRD, respectively. Ash fusion temperatures (AFTs) under light reducing atmosphere and the pressure-drop temperature(PDT) were tested. SEM-EDX was also employed to observe the melting behavior. It is found that the softening temperature of Jincheng coal is 1501℃, while that of its density fractions changes from 1292℃ to 1600℃. The sintering temperature changes from 833℃ to 943℃, while that of the raw coal ash was 885℃. The study of mechanism shows that the difference of melting and sintering behaviors is caused by the segregation of chemical composition in different density samples, mainly Al₂O₃, Fe₂O₃ and SO₃.