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Nanostructure of block copolymer in thin film could be fabricated via self-assembly process having preferential or random microdomain orientation.A parallel orientation of cylindrical microdomains would be favourable with preferential interactions of the substrate with one block,and/or the large difference in surface energies of the two blocks,while the perpendicular orientation is achievable by controlling the interfacial and surface interactions.We investigated water permeability of cylinder-forming polystyrene-block-poly(methyl methacrylate)(PS-b-PMMA)films using neutron reflectivity(NR)by comparing NR data of homo-polystyrene(h-PS)and homo-polymethacrylate(h-PMMA).The microdomain orientation dependence on water permeability will be discussed in terms of density distribution of each block.With the density distribution of each block,mechanism of water permeation in perpendicularly-oriented block copolymer thin film will be investigated.