Polymerization-induced self-assembly (PISA) is proven to be a powerful approach of in situ synthesis of block copolymer (BCP) nanoassemblies, and polymerization conditions are found to be correlative to the block copolymer morphology. In this study, three PISA formulations, e.g., the poly(ethylene glycol) macro-RAFT agent mediated dispersion RAFT polymerization, seeded dispersion RAFT polymerization, and seeded emulsion RAFT polymerization, are comparatively investigated. Our results reveal that dispersion RAFT polymerization undergoes much slower than other two PISA formulations of seeded dispersion RAFT polymerization and seeded emulsion RAFT polymerization. Besides, the results reveal that the BCP morphology of poly(ethylene glycol)-block-polystyrene (PEG45-b-PS) produced via three PISA cases is much different. That is, dispersion RAFT polymerization affords vesicles, seeded dispersion RAFT polymerization affords the mixture of vesicles and porous nanospheres, and seeded emulsion RAFT polymerization affords porous nanospheres of PEG45-b-PS. The reason for formation of porous nanospheres by seeded RAFT polymerization is discussed, and the fed styrene monomer swelling the seeded vesicles is ascribed. Our study clarifies how the PISA procedures affect the morphology of BCP nanoassemblies, and it is expected to be effective to prepare BCP nanoassemblies with interesting morphology.