The effects of substituted groups on the characteristics and water vapor sorption property of polyhedral oligomeric silsesquioxane (POSS)-containing methacryl polymers were investigated. A broad halo was observed in the X-ray diffraction profile, which suggests that all membranes were amorphous structure. Interestingly, the smaller volume of substituted groups tended to induce the larger deformation of POSS structure resulting from the packing of polymer segments. The introduction of ring-substituted POSS unit improved their thermal stability due to the increase in the rigidity. The sorption behavior for all POSS-containing polymer membranes obeyed the dual-mode model at lower relative pressure and then upturns at higher pressure. The phenyl-POSS shows the high solubility due to the large excess free volume and suppression of hydrophobic POSS unit covered by the stacking effect. Based on these results, the introduction of POSS unit enhances the hydrophobic property, while the polarity of substituted groups strongly affects the water vapor clustering.