Amphiphilic diblock copolymer with different poly(ε-caprolactone) (PCL) block length were synthesized by ring-opening polymerization of ε-caprolactone in the presence of monomethoxy poly(ethylene glycol) (mPEG-OH, MW 2000) as initiator, using stannous octoate catalyst. The self-aggregation behavior and microscopic characteristics of the diblock copolymer self-aggregates, prepared by dialysis method, were investigated by ¹H NMR, dynamic light scattering (DLS), fluorescence spectroscopy. The PEG-PCL block copolymer formed self-aggregates in an aqueous environment by intra- and/or intermolecular association between hydrophobic PCL chains. The microscopic characterization of the PEGCL self-aggregates revealed that the hydrophobic/hydrophilic balance, resulting from different PCL block length, played a crueial roles of ditermination of the physicochemical properties of them.