Diblock copolymers with different poly(epsilon-caprolactone) (PCL) block lengths were synthesized by ring-opening polymerization of epsilon-caprolactone in the presence of monomethoxy poly(ethylene glycol) (mPEG-OH, MW 2000) as initiator. The self-aggregation behaviors and microscopic characteristics of the diblock copolymer self-aggregates, prepared by the diafiltration method, were investigated by using H-1 NMR, dynamic light scattering (DLS), and fluorescence spectroscopy. The PEG-PCL block copolymers formed the self-aggregate in an aqueous environment by intra-and/or intermolecular association between hydrophobic PCL chains. The critical aggregation concentrations of the block copolymer self-aggregate became lower with increasing hydrophobic PCL block length. On the other hand, reverse trends of mean hydrodynamic diameters were measured by DLS owing to the increasing bulkiness of the hydrophobic chains and hydrophobic interaction between the PCL microdomains. The partition equilibrium constants (K-upsilon) of pyrene, measured by fluorescence spectroscopy, revealed that the inner core hydrophobicity of the nanoparticles increased with increasing PCL chain length. The aggregation number of PCL chain per one hydrophobic microdomain, investigated by the fluorescence quenching method using cetylpyridinium chloride as a quencher, revealed that 4-20 block copolymer chains were needed to form a hydrophobic microdomain, depending on PCL block length. (c) 2006 Wiley Periodicals, Inc.