A series of thermo-responsive gating membranes with controllable length and density of grafted polymer chains are prepared by grafting poly(N-isopropylacrylamide) (PNIPAM) chains in the pores of anodic aluminum oxide (AAO) porous membranes with atom-transfer radical polymerization (ATRP) method. Effects of grafting temperature, grafting time, feed concentration of monomer, and density of -Br on the grafting yields of AAO-g-PNIPAM membranes are systematically investigated. The thermo-responsive characteristics of AAO-g-PNIPAM gating membranes with different lengths and densities of grafted PNIPAM chains are investigated by tracking the diffusional permeation of vitamin B12 at temperatures below and above the lower critical solution temperature. The results show that thermo-responsive characteristics of the AAO-g-PNIPAM membranes are heavily affected by both the length and density of grafted PNIPAM chains in the membrane pores, and the effect of the length of grafted PNIPAM chains is more significant than that of the density. The results demonstrate that, to obtain desired performances of gating membranes, it is essential to design and fabricate grafted PNIPAM chains with proper length and density. (C) 2009 Elsevier B.V. All rights reserved.