Abstract: Adopting the Langmuir monolayer adsorption theory, the experimental research was done for adsorbing methane under the different particle size conditions. By using the WY-98A gas constant determinator and WFC-2 initial speed determinator of methane diffusing, it made the qualitative and quantitative analysis for the curve of isothermal adsorption, adsorption constants a and b and initial speed of methane diffusing △p for different coal samples under the different particle size conditions, and obtained the relationship of adsorption constant a and initial speed of methane diffusing △p with the particle size change. The results showed that adsorption constant a would present periodic change with particle size varying, and the initial speed of methane diffusing △p took on the variation law of logarithmic function.