Multicomponent block copolymer nanoassemblies are constructed by two or more than two block copolymers. Herein, multicomponent block copolymer nanoparticles constructed with poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS) and poly(ethylene glycol)-block-polystyrene (PEG-b-PS) are synthesized by RAFT dispersion polymerization of styrene employing poly(ethylene glycol) trithiocarbonate (PEG-TTC) as the macromolecular chain transfer agent (macro-CTA) in the presence of P4VP-b-PS nanoparticles, which are presynthesized by ATRP dispersion polymerization. It is found that multicomponent block copolymer nanoassemblies instead of the mixture of the PEG-b-PS nanoassemblies and the P4VP-b-PS nanoparticles are formed under suitable conditions. The possibility how multicomponent block copolymer nanoassemblies are synthesized is discussed, and the evolution of the size and/or morphology of the multicomponent block copolymer nanoassemblies is investigated.