The aim of this study is to understand details about the sample size influence on the CH4 and CO2 sorption behaviors of coals. Two anthracites (YQ and QS) were selected, ground and sieved. Four size fractions (from powder size -0.354 mm to the lump size 4-10 mm) were tested. The manometric method was used to determine the CH4 and CO2 sorption isotherms and kinetics. The unipore diffusion model was selected for sorption kinetic modeling. Although the compositions of the different size fractions generally exhibit slight variations the isotherms with different sample basis (air-dried, ash-free or dry-ash-free) are not apparently different, except for the largest size YQ sample that contains significantly higher amounts of minerals. However, the "ash correction'' or the "ash-moisture correction'' made the sorption isotherm of the largest size YQ sample deviate from the main trend. The predicted effective diffusion coefficients of CH4 and CO2 for both coal samples increase, however, unexpectedly with the increase of the sample size. It is probably due to the differences in the mineral contents, the improper estimation of sample size, and the differences in pore structures. (C) 2011 Elsevier Ltd. All rights reserved.