Complete quantitative data of the chemical (proximate, ultimate, and ash analyses) and mineral (in low-temperature ash (LTA) and various high-temperature ashes (HTA)) compositions of 21 coals were used to investigate the modes of occurrences and high-temperature behaviors of the minerals in coals and their influence on ash fusibility. The common minerals present in the low-temperature ashes (LTA) are kaolinite, quartz, muscovite, calcite, gypsum, pyrite, and siderite. The samples were divided into two groups according to the hemispherical temperature for a comparative study of the behavior of mineral matters. Results show that the average number of mineral species (ANMS) and amorphous substances (AS) in the LTAs of the two groups are essentially the same. The ANMS in both the low and high (ash fusion temperatures, AFT) ash samples go through the same tendency of a slight reduction at first, an increase, and finally, a significant reduction. As the temperature increases, the ANMS in the low-AFT ash is initially higher and then lower than the high-AFT ash, whereas the tendency of the AS is quite the opposite. The ash melting process is divided into three stages, and the AFTs are related to different degrees of the eutectic stage.