In this letter, we describe the surface modification of a polyethersulfone (PES) membrane by deposition of a nanoporous parylene film. The glycerin vapor during parylene deposition prevented parylene from forming over the pores. Water contact angle measurements and diffusion measurements revealed that parylene could be coated onto the PES membrane while keeping some pores open when the amount of dimer was properly controlled. The parylene-coated membrane prepared by our modified deposition process gave higher diffusion values than that prepared conventionally. Moreover, the platelet activation and adhesion were suppressed after coating parylene; this result indicated that the parylene-coated PES membranes showed improved biocompatibility. This advancement has a potential benefit for ultrafiltration applications.