To investigate thermosensitive polymeric nanoparticle, amphiphilic block copolymers of poly(N-isopropylacrylamice)-b-poly(epsilon-caprolactone) (PNPCL) with different PCL block lengths were synthesized by hydroxy-terminated poly(N-isopropyoacrylamide) (PNiPAAm) initiated ring opening polymerization of E-caprolactone. Owing to their amphiphilic characteristics, the block copolymers formed self-assembled polymeric nanoparticles in aqueous milieus with thermosensitive PNiPAAm shell compartment. The characterizations of the nanoparticles revealed that the PNPCL nanoparticles showed PCL block length dependent physicochemical characters such as particle sizes, critical aggregation concentrations, and core hydrophobicities. Moreover, the thermosensitive PNiPAAm shells conferred unique temperature responsive properties such as phase transitions with temperature elevation over its lower critical solution temperature (LCST). The temperature induced phase transition resulted in the formation of PNiPAAm hydrogel layer on the PNPCL nanoparticle surface. The drug release tests revealed that the formation of thermosensitive hydrogel layer resulted in the enhanced sustained drug release patterns by acting as an additional diffusion barriers. Therefore, the introduction of thermosensitive polymers on polymeric nanoparticles might be a potential approaches to modulate drug release behaviors. (c) 2006 Elsevier Ltd. All rights reserved.