The permeation behavior of carbon dioxide in a melt cis-polyisoprene (cis-PI) membrane has been studied by a psuedo-nonequilibrium molecular dynamics (MD) simulation. The three-phase model, gas/cis-PI-membrane/vacuum, was constructed. The adoption of the virtual liquid (VL) molecules which have no interaction with gas molecules and also the artificial potential walls between the interface enabled the permeation simulation by making the concentration slope of the gas molecules. Time enhancement of the gas molecules which go across the membrane showed a linear relation after attaining a steady state. The permeation coefficient was directly obtained from the time lag analogous to the experiment at 373 K. (C) 2002 Elsevier Science B.V. All rights reserved.