Abstract: In order to describe the adsorption characteristics of coal on methane more accurately, based on conventional isothermal adsorption test, methane adsor ption capacity of coal was modified, and the calculation of apparent and true adsorption volumes were introduced, besides, the differences between apparent and true a dsorption volume were compared, and the effect of temperature on such differences was analyzed, according to the true adsorption volumes, and then the fitted formula s between adsorption constant and temperature was established. The results showed that: the differences between apparent and true adsorption volumes became muc h more significant with the rising temperatures. When temperature reached 90°C, the error between apparent adsorption and true adsorption volume was maximum, 8%. The adsorption ability of coal was proportional to the specific surface, when temperature rose, due to the increase of specific surface area, the adsorption ability of coal presented a linear growth with temperature. Such research results can help to understand the adsorption properties of lump coals, and provided appropriate guidance in exploitation technique of coalbed methane ( CBM) .