Change in tensile strength of a biomass ash powder bed measured by a split-cell type tester at a variety of temperatures up to 900degreesC was discussed corresponding to the microscopic behavior of ash particles observed by a field emission scanning electron microscope (FE-SEM) with a computer-controlled stage, a heat treatment chamber and an energy dispersive X-ray spectrometer. Tensile strength of a biomass powder bed increased with temperature by two different steps. In temperatures ranging from 400 to 700degreesC, the powder bed had liquid-phase bridging between particles, which caused the relatively large fracture distance in fracture curves. FE-SEM observation showing rounded ash particles after heat treatment at 500degreesC supported this phenomena. On the other hand, at temperatures higher than 700degreesC, powder beds showed brittle fracture, which means the absence of any liquid bridge due to evaporation. FE-SEM observation revealed successive evaporation from ash samples in this temperature region.