In this paper, we report a facile large-scale synthesis of the "smallest" (i.e., unimolecular) polymeric Janus nanoparticles from a polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO or SVEO) triblock copolymer by efficient intramolecular cross-linking of the middle P2VP block using 1,4-dibromobutane (DBB) in a common solvent, N,N-dimethylformamide (DMF), due to effective steric shielding of PS and PEO end blocks. Size-exclusion chromatography results indicated that intramolecular cross-linking of the middle P2VP block could take place when the polymer concentration was relatively high (20 mg/mL) and/or the DBB-to-2VP molar ratio was high. Dynamic and static light scattering experiments confirmed the unimolecular form for these polymeric Janus nanoparticles. After intramolecular cross-linking, DMF changed from a good to a slightly poor solvent, as evidenced by the negative apparent second Virial coefficient for the unimolecular Janus nanoparticles determined by SLS. As a result, concentration-dependent self-assembly in DMF was observed. At low concentrations (<2.0 mg/mL), the majority of the unimolecular polymeric Janus nanoparticles existed in the unimolecular form. When the concentration gradually increased, the unimolecular polymeric Janus nanoparticles started to aggregate into supermicelles (R-h = 50-100 nm), where PS formed the supercore and PEO formed the corona with cross-linked P2VP nanoparticles in between. The amphiphilic nature of these unimolecular Janus nanoparticles will enable us to study programmable and hierarchical self-assembly of asymmetrically modified polymeric nanoparticles in various solvents.