In this study, we present a new route to synthesize an intelligent polymer nanocapsule with an ultrathin membrane based on surface-initiated reversible addition-fragmentation chain-transfer polymerization. The key concept of our report is to use pH-responsive polydiethylaminoethylmethacrylate as a main membrane-generating component and a degradable disulfide bond to cross-link the membrane. The permeability of membrane, tuned by adjusting pH and using different lengths of the cross-linkers, was proven by showing a dramatic swelling behavior of the nanocapsules with the longest cross-linker from 560 nm at pH 8.0 to 780 nm at pH 4.0. Also, due to the disulfide cross-linker, degradation of the capsules using GSH as reducing agent was achieved which is further significantly promoted at pH 4.0. Using a rather long-chain dithiol cross-linker, the synthesized nanocapsules demonstrated a good permeability allowing that an enzyme myoglobin can be postencapsulated, where the pH controlled enzyme activity by switching membrane permeability was also shown.