An amphiphilic multiblock copolymer, [poly(4-vinylpyridine)-b-polystyrene-b-poly(4-vinylpyridine)]_n (P4VP-PS-P4VP)_n bearing trithiocarbonate moieties (–S–CS–S–), was used as capping agent to fabricate the functional copolymer-capped Au nanoparticles. Due to the amphiphilic character of the multiblock copolymer, the (P4VP-PS-P4VP)_n-capped Au nanoparticles were able to be entrapped and hence form a stable monolayer thin film at the (DMF–H₂O)/diethyl ether interface. Note that DMF–H₂O is a good mixed solvent for P4VP and a nonsolvent for PS, whereas diethyl ether is a good solvent for PS and a nonsolvent for P4VP. A similar procedure can be applied to prepare monolayer thin film of the (P4VP-PS-P4VP)_n-capped Ag nanoparticles at the (DMF–H₂O)/diethyl ether interface, which exhibits enhanced surface Raman signal. We further demonstrate that the stable monolayer of (P4VP-PS-P4VP)_n-capped Au nanorods can also be fabricated by using the ligand-exchange method.