Free-energy related properties for penetrant sorption (water, methane, argon, oxygen, nitrogen, ethane, propane, and carbon dioxide) in amorphous polyethylene have been evaluated by a test-particle-insertion method with the excluded volume map sampling (EVMS). Two model systems, one a long chain model composed of 4 linear chains each having 1002 carbon atoms and another a short chain model composed of 167 molecules of C24H50, were compared in relation to the penetrant solubility at 298 K. The time-dependent molecular configurations were obtained by a molecular dynamics (MD) simulation under constant particle number, constant pressure, and constant temperature (NPT) conditions. The solubility coefficient obtained by the EVMS method was larger in the order, C3H8> C2H6> CO2> CH4> Ar > O-2> N-2, similar to the experimental results, and the absolute values were also in agreement with the experimental value. In the long chain host-molecules, reflecting the higher density, the solubility was about 0.85-0.90 times smaller for most penetrants tested, while ethane and propane showed higher solubility in contrast. Analysis of the averaged unoccupied volume fraction and their distribution in both systems revealed that big holes, in which a sphere with a diameter larger than 5.0 Angstrom can be introduced, were formed in the long chain model irrespective of the denser structure. These findings resulted in higher solubility of a larger penetrant such as ethane and propane in the long chain matrix.