The self-aggregation behavior and microscopic characteristics of the PEG-b-PCL copolymer self-aggregates, prepared by solvent evaporation method, were investigated by ¹H NMR, dynamic light scattering (DLS), and fluorescence spectroscopy. The PEG-PCL block copolymer formed nano-sized self-aggregates in an aqueous environment by intraand/or intermolecular association between hydrophobic PCL chains. Then, drug loading and its release behavior of clonazepam loaded PEG-b-PCL nanoparticles were investigated. The result revealed that hydrophobic-hydrophilic balances played a crucial role for drug loading and its release behaviors.