Functionalized polymer nanoparticles have been synthesized through an intramolecular cross-linking reaction with polystyrene as the backbone and an amino group as the functional moiety. The intramolecular cross-linking reaction was performed under ultradilute conditions, and the crosslinked polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and atomic force microscopy (AFM). The availability of the amino moiety in the nanoparticle for further functionalization was confirmed by its reaction with trimethylacetyl chloride.