Polyimide is deposited on the surface of nanoporous anodized alumina by atomic layer deposition (ALD) using pyromellitic dianhydride and ethylenediamine as the two precursors. Such ALD reactions directly produce polyimide rather than the intermediate polyamic acid. The precursor exposure duration plays an important role in determining the growth rate of PI and consequently the reduction of effective pore sizes. ALD with short precursor exposure confines the deposition predominantly around the pore openings of the alumina substrates, producing an asymmetric structure in the form of thin-film composites. This structure efficiently reduces the pore sizes of alumina, and as a result the retention of the membrane is significantly improved. A moderate cycle number of 50 remarkably increases the rejection of the membrane from nearly none to 82% at an acceptable expense of a reduction of 63% in water permeability. Deposition with long precursor exposure results in uniform coating into deep pores of the membranes whereas the effect in tailoring pore sizes and filtration performances are not as pronounced as deposition with short precursor exposure. This work is expected to be adopted to fabricate highly permeable and selective membranes of different materials by leveraging the deposition behavior of ALD on appropriate substrates.