Co-firing has been adopted as a major approach for the efficient use of biomass as an energy source in large scale for two decades. Such a practice contributes to the decarbonisation in power generation sector and therefore to the mitigation of CO₂ emission. However, due to the differences in terms of thermal behaviors between biomass and coal, the co-firing of some biomasses with coal still presents many engineering problems, such as excessive fouling and slagging, which subsequently result in deteriorated boiler efficiency. The understanding of the transformation of minerals upon heating and how minerals originated in biomass affect ash fusion characteristics is of some significance for both design and operation purposes. In this study, low temperature ash (LTA) samples of biomass and coal were prepared and analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled energy diffraction spectrum (EDS), the results of which were compared with their corresponding high temperature ash (HTA) samples. The transformation of minerals upon heating was therefore studied. The ash fusion characteristic temperatures of ash samples derived from different biomass and coal blends were investigated. The ash fusion characteristic temperatures show some additive behaviors.