We have investigated the effect of chain flexibility on the sieving ability of polymers by means of a molecular dynamics simulation method. As the size of penetrants and the rigidity of chains increase, the diffusion coefficients decrease. The more rigid chains give a greater difference between the diffusion coefficients of smaller and larger penetrants, which results in an improvement of selectivity. In the case of the penetrant whose size is smaller than that of the polymer segment, it is observed that the condition with an optimum selectivity and permeability exists resulting from a competition between the effect of the redistribution of dynamic free volume and that of the amount of static free volume on the diffusion of penetrants. One must take both the available free volume and the chain flexibility into account when designing a membrane for gas separation process.