To invest thermosensitivity on polymeric amphiphiles, the poly(N-isopropylacrylamide)-b-poly(εcaprolactone) block copolymers (PNPCL) with different PCL block length were synthesized by hydroxyl-terminated poly(N-isopropylacrylamide) (PNP-OH) initiated ring opening polymerization of ε-caprolactone. Thermosensitive nanaparticles formed by self-assembly of PNPCL in aqueous environment, were prepared by a solvent evaporation method and their physicochemical characteristics were prepared by a solvent evaporation method and their physicochemical characteristics were investigated. The nanoparticles showed PCL block length dependent physicochemical properties such as particle size, critical aggregation concentration, and inner core hydrophobicity. Furthemore, the PNPCL nanoparticles showed typical thermosensitivities, such as particle size and optical transmittance changes by increasing temperature, owing to the unique extended coil to globule transition of PNP shells. Then, the thermosensitive nanoparticles were introduced into drug delivery systems using hydrophobic model drug of clonazepam (CNZ). The results of drug release behaviors of CNZ loaded nanoparticels revealed that the longer PCL block length thermosensitive PNP shells played crucial role for the drug release behaviors.