Poly(methacrylic acid) was grafted on a nanoporous polycarbonate membrane using the glow-discharge method. Atomic force microscopy showed that the pore size is dependent upon pH in such a way that at low pH the graft chains are protonated and contracted to open the pore, whereas at high pH, the graft chains became extended, thus reducing the pore size. Water permeation through the graft membrane is regulated by the pH value. The permeation rate is low at high pH value and high at low pH. This phenomenon corresponds to the pore size.