A series of nanoporous membranes prepared from polyethylene-block-polystyrene were applied for size-selective diffusion of glucose and albumin molecules. Millimeter-sized test cells for characterization of such molecular diffusions were designed assuming an implantable glucose sensor. The prepared nanoporous membrane exhibits excellent flexibility and toughness compared to conventional nanoporous membranes of brittle alumina. Pore size of the membranes could be controlled from 5 to 30 nm by varying preparation conditions. All of these nanoporous membranes prepared in this study let glucose pass through, indicating a continuous pore connection through the entire thickness of the membrane in a few tens of micrometers. In contrast, membranes prepared under optimum conditions could perfectly block albumin permeation. This means that these vital molecules having different sizes can be selectively diffused through the nanoporous membranes. Such a successful combination of size selectivity of molecular diffusion in nanoscale and superior mechanical properties in macroscale is also beneficial for other devices requesting down-sized manufacture.