Abstract: The slagging mechanism of the Jincheng coal in the ash-agglomerate fluidized bed gasification was explored. A typical Jincheng coal sampled from Shanxi, China was prepared into different particle size and the XRF, XRD, AFT, SEM and FactSage^TM were employed to study the influence of particle size on the ash content, chemical and mineral composition and ash fusibility. The results indicate that the ash chemical composition and AFTs vary little with size below 6 mm. For the pulverized samples below 0.2 mm, there are apparent differences in the chemical composition and mineral composition, especially in Fe₂O₃. Both AFT results and SEM observations show that the sample with particle size less than 45 μm has a significantly higher melting temperature than the other three samples. The slag contents calculated with FactSage^TM are consistent with the AFT results and SEM observations, which indicates that the change of ash melting behavior with particle size results from the change in chemical composition. The ternary phase diagram of SiO₂-Al₂O₃-Fe₂O₃ can be used successfully to explain the mechanism of the change of ash melting behavior with particle size for Jincheng coal.