To investigate thermosensitive polymeric nanoparticle, amphiphilic block copolymers of poly(N-isopropylacrylamide)-b-poly-(ε-caprolactone) (PNPCL) synthesized by hydroxy-terminated poly(N-isopropylacrylamide) (PNiPAAm) initiated ring opening polymerization of ε-caprolactone. 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. 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.