Polymer membranes are used in a wide range of categories such as gas separation, CO2 catalysis, and wastewater treatment. The preparation method exerts a crucial influence on the structure and properties of the membrane. In this work, three block copolymer nanoassemblies have been initially prepared by reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization via polymerization-induced self-assembly (PISA), and then they are used to prepare block copolymer nanoassembly membranes by fusion of the block copolymer nanoassemblies with different morphologies, such as nanospheres, vesicles, and worms. The block copolymer nanoassembly membranes have many particle interfaces inside, which are different from the block copolymer membranes prepared by casting a polymer solution. Consequently, the mechanical property of the block copolymer nanoassembly membrane is inferior to that of the block copolymer membranes. Heating the block copolymer nanoassembly membrane above the glass transition temperature (Tg) of the block copolymer can promote interparticle fusion and therefore increase its mechanical strength. It is found that the typical membrane formed by block copolymer worms has a much higher CO2 permeability than the block copolymer membrane (754 vs 287 Barrer), suggesting less fusion of block copolymer worms to lead particle interfaces during membrane formation. The present study reveals how the polymer particle morphology affects the structure and properties of the block copolymer nanoassembly membranes, and this is believed to be useful for designing polymer membranes.