Star shaped block copolymer POSS-(PMMA-b-PS)₈ was synthesized by a two step process of atom transfer radical polymerization (ATRP) with eight functionalized polyhedral oligomeric silisesquioxane POSS-(Cl)₈ as core and poly(methyl methacrylate-b-polystyrene) as arm. The POSS-(PMMA-b-PS)₈ was then sulfonation treated to produce hybrid polymer POSS-(PMMA-b-SPS)₈, which was finally used as the polymer matrix for making proton exchange membranes (PEMs). The examination of conductivity as function of relative humidity for PEMs of high and low hydration status respectively indicated that with longer SPS block length exhibited higher proton conductivity for the PEMs of low hydration status with the same λ i.e. the number of water molecular coupled to sulfonic acid groups. TGA analysis showed that two kinds of PEM all exhibited higher water retention capacity and higher initial decomposition temperature. A well-connected ionic domains in PEM with longer SPS block could be observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The features of molecules motion of chain segments and spins relaxation time T₂ for the PEMs of low hydration status were analyzed by low field nuclear magnetic resonance, and it is found that the well connected ionic domains could be observed also in the PEMs with longer SPS block, which exhibited higher proton spin-diffusion coefficient, therewith higher proton conductivity by low relative humidity.