Sorption of gas molecules into a polymer film was studied by molecular dynamics (MD) computer simulation. The system is composed of 200 O-2 molecules and 60 polyethylene molecules consisting of 100 CH2 groups. We used the Rigby-Roe model for the polymers. The simulation was executed in a wide temperature range. The film has crystalline structure at 180 and 300 K, while it seems to melt film at 500 K. More absorption is observed at higher temperatures due to the film structure. Adsorption is stronger at lower temperature because adsorbed molecules stay on the surface for a longer period. From the number of absorbed/adsorbed molecules, the isothermal sorption curve was determined.