A series of copolymers of phenolphthalein with 4,4’-biphenyl dicarbonyl chloride and bis[p-(chlorocarbonyl)phenyl]dimethylsilane were synthesized, and their flexible films were cast readily. The gas transport properties for hydrogen, oxygen, nitrogen, carbon dioxide, and methane in membranes were measured by a low pressure method at 30-degrees-C and 1 atm. As the solubility-diffusion process analysis, how the molar content of the silane segment on the polymer backbone affects the behavior of gas transport through an aromatic polyester was studied. It is sure that the packing density is an important factor for determining the gas diffusivity in a polymer. With the increasing molar content of the silane segment, the packing density reduces and the gas diffusivity in the aromatic polyester increases. Contrary to that, gas solubility reduces. Meanwhile, both diffusivity selectivity and solubility selectivity decrease. The result is that gas permeability rises and permselectivity for hydrogen over nitrogen, oxygen over nitrogen, and carbon dioxide over methane in a membrane of an aromatic polyester reduces with the introduction of a silane segment on the polymer backbone. The correlation between the concentration of the carbonyl group and the gas solubility selectivity is discussed in this paper. At the end, we compare the gas transport properties of those aromatic polyesters with common commercial polymer materials used as the separation membrane.