A series of granular activated carbon (GAC) samples with similar surface chemical properties but different pore structures were prepared from anthracite. The maximum adsorption capacities of the prepared CO2, CH4, and N2 at 298 K and 2.0MPa were 4.27mmol/g, 2.54mmol/g, and 1.46mmol/g, respectively, and the adsorption selectivity parameters, i.e., αCH4,N2 and αCO2,CH4, were 3.23 and 3.06, respectively. By using the GAC with the optimum pore size as adsorbent, the concentration of methane in the nitrogen-methane (CH4/N2) mixture was concentrated from 30% to 63.5% via a single-column single-cycle pressure swing adsorption (PSA) process. The pore size distribution of the GAC samples was dominated by micropores, with specific surface area in the range of 330-500m2/g and micropore volume in the range of 0.12-0.19 cm3/g. Although the specific surface area and pore volume of micropores played an important role in the separation performance, the pore size distribution was found to be the decisive factor. In particular, the micropores with sizes in the range of 5.0-10.0 Å were the main factor affecting the concentrating effect of CH4 or CO2 by GAC.