Understanding the sorption behavior of gas in organic-rich sedimentary rocks and, more specifically, recognizing the adsorption properties of methane in coal and crucial steps for evaluating the coalbed methane (CBM) gas-in-place content, gas quality, and CBM recovery potential. However, the adsorption affinity of coal on methane has not been previously considered. This paper introduces the isosteric heat of adsorption in Henry's region, renamed the mean isosteric heat of adsorption, as means to evaluate the adsorption affinity of coal on methane. 18 group isothermal adsorption tests for methane in three different coals were conducted from 243.15 to 303.15 K The mean isosteric heat of adsorption for anthracite, lean coal, and gas-fat coal is -23.31, -20.47, and -11.14 kj/mol, respectively. The minus signs indicate that the adsorption is an exothermal process. The mean isosteric heat of adsorption is independent of the temperature from 243.15 to 303.15 K and shows the overall heterogeneous property of different coal. Therefore, the mean isosteric adsorption of heat can serve as a quantified index to evaluate the coal adsorption affinity on methane.