To evaluate the adhesive property of coal flyash particles, a high temperature shear tester capable of measuring up to 950 degreesC has been developed. Shear test can be done at different gas conditions such as air, N-2, CO2 and N-2-CO2 mixture at different ratio and all manipulations can also be done from the outside the furnace. Shear stress of flyash particles from pulverized coal combustion but different lime addition was measured from ambient to 950 degreesC. As a result, adhesive force and internal friction angle are found to change slightly with temperature and gas conditions up to 700 degreesC. However, a noticeable increase in adhesive force was observed at temperature higher than 800 degreesC, especially higher than 850 degreesC. Furthermore, an interesting phenomenon was observed when limestone was tested under CO2 containing condition, i.e., adhesive force of lime contains flyash increases at most about three-halves depends on CO2 content of gas.