This paper analyzes the increasing mechanism of cohesive behavior of ash powders at elevated and high-temperature conditions. Model ash powders were prepared from fine pure amorphous silica powder coated and heat-treated on the surface with 0.5 wt % sodium and/or potassium. The cohesive behavior and deformation properties of model ash powder beds were determined by using a new split-type tensile strength tester of powder beds. The adhesion behavior of fly ash collected in a pulverized coal combustion (PC) and a pressurized fluidized bed combustion (PFBC) system was compared with the results of model ash powder samples. In the high-temperature range above 800 degreesC, a rapid increase of tensile strength and plastic rupture deformation occurred in both the natural ash samples and the model ash powder samples prepared from alkali metal and pure silica powder. It is suggested that the coating alkali metal reacted with the amorphous silica phase and formed a small amount of low-melting-point eutectic materials, such as K2O.4SiO(2). Since these eutectic materials formed a small amount of liquid phase at interparticles at about 800 degreesC, the tensile strength and plastic rupture deformation rapidly increased at high temperatures above 800 degreesC.