A Boundary-Driven type Non-Equilibrium MD (BD-NEMD) technique combined with the mu VT-MC and NpT-Gibbs MC methods has been developed to simulate membrane permeation of gas mixtures. By using the new technique, simulations were carried out on permeation of binary mixtures, which consist of two kinds of LJ fluids differing in the LJ energy parameter and the molar mass. Two important results were obtained; (1) the permselectivity is affected by two factors, the adsorption equilibrium selectivity and the difference in molecular diffusivity, and (2) it increases as the weakly adsorptive molecules diffuse more slowly.