To analyze the phenomenon of the increase in cohesive force between fine particles under elevated temperature conditions, a new split-type tensile-strength tester of powder beds for high temperature was developed. Fused silica glass with low thermal expansion coefficient was used as the material for the suspended cell of a commercial split-type tensile-strength measuring device at room temperature. The powder samples used were pure fine silica and two kinds of fly ash powders collected in a pulverized coal (PC) combustion and a pressurized fluidized coal combustion systems. Under conditions of relatively low temperatures below 1000 K, the adhesive force of all powders increased gradually in proportion to the temperature. Rapid increases of tensile strength of both fly ash powder beds were observed in the high-temperature range (above I 100 K). Based on the results of thermo-mechanical analysis (TMA), it was estimated that a viscous flow sintering mechanism controlled this rapid increase phenomenon of adhesive behavior of ash powders at high-temperature conditions above I 100 K. On the other hand, using FT-IR analysis, it was demonstrated that the increase of the van der Waals force with the change of surface molecular structure controlled the stickiness of pure silica and both ash powders at relatively low-temperature conditions below 1000 K.