To prepare nanoparticles, which are thermosensitive, biocompatible, and biodegradable, the well-defined PNIPAAm-PCL and PEG-PCL block copolymers were synthesized by ring-opening polymerization of ε-caprolactone using mPEG-OH and PNIPAAm-OH and characterized by ¹H-NMR and GPC. Thermosensitive nanaparticles according to various ratios of PNIPAAm-PCL and PEG-PCL were prepared by a solvent evaporation method. The nanoparticles showed composition dependent physicochemical properties such as lower critical solution temperature, particle size, and critical aggregation concentration. Then, the nanoparticles were introduced into drug delivery systems using hydrophobic model drug of clonazepam. The drug release behaviors from nanoparticles were investigated.