In this work, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was modified by trimethylchlorosilane and triphenylchlorosilane, and the gas permeation properties in silyl-substituted PPO were studied by changing the substitution group, substitution position, and substitution value. The substituent group on phenylene of the polymer main chain affects the gas permeability in modified PPO more significantly than that on the methylside group. Gas permeability and some permselectivity in trimethylsilyl-substituted PPO increase, but the permeability and permselectivity in triphenylsilyl-substituted PPO both decrease as the substitution value increases. Because of similar values of gas solubility but various diffusivities among PPO and modified PPO, the difference in gas permeabilities in the polymers is caused mainly by the change in their gas diffusivities. The correlation between gas permeability and polymer molecular structure is summarised, and it is inferred that the gas permeability in the polymer may increase but permselectivity is not reduced if a moderate content of a bulky free group is introduced into a suitable position of the molecular backbone.