Abstract: Nanocapsules have been widely applied in drug delivery and controlled release. In this work, a class of polydopamine/poly(ethylene glycol) (PDA/PEG) composite nanocapules were developed with a zeolite imidazole frameworks (ZIF-8) as the template. PDA was first coated onto the as-synthesized ZIF-8 nanoparticles by mussel-inspired oxidation and self-polymerization of dopamine in alkaline aqueous condition. PEG chains were then grafted onto the PDA-coated ZIF-8 particles by the Michael addition or Shiff-base reaction between amine-terminated PEG and PDA coatings. Further, the ZIF-8 templates were etched and removed by the decrease of pH value of the suspension, and the PDA/PEG composite nanocapsules were obtained. The chemical compositions, crystal structures, physical morphologies, particle size distribution and charged characteristics of the intermediates PDA@ZIF-8 and PDA/PEG@ZIF-8 nanoparticles were characterized in detail. The experimental results showed that the PDA/PEG@ZIF-8 composite nanoparticles were prepared successfully and the average particle size was about 80 nm. The average size of the PDA/PEG composite nanocapsules reduced to about 34 nm due to the etching and removal of ZIF-8 templates. The introduction of PEG brushes was advantageous to improve the dispersion and stability of nanoparticles and nanocapsules in water. This work develops a new approach for the preparation of polymer nanocapsules used in drug delivery and controlled release.