Natural gas is one of the important principle sources of energy to the world. The impurities; carbon dioxide, hydrogen sulfide and heavy hydrocarbon from natural gas facing separation process before delivered to the end user markets. Currently, membrane processes have been broadly applied since it provides operating and cost advantage rather than other technologies. In this work, the behaviour of polymer and penetrant is predicted using a conventional mathematical model. Dual mode sorption and partial immobilization of the hole population transport model are used with the assumption of local equilibrium relaxed. Parameter estimation method is used to evaluate the separation of CO 2, and CH 4 impurities mixture. The effect of operating pressure on the sorbed concentration, diffusion, permeability coefficient and selectivity of penetrant in the membrane are studied. The result shows the sorption and permeation properties are well fitted to a dual mode model. The permeability coefficient of pure gas is higher than mixture gas measurement due to competition effects between the penetrant. © 2011 IEEE.