Thermosensitive nanoparticles were prepared by self assembly of two different poly(ε-caprolactone) based block copolymers of poly(N-isopropylacrylamide)-b-poly-(ε-caprolactone) (PNP-PCL) and poly(ethylene glycol)-b-poly-(ε-caprolactone) (PEG-PCL). The self-aggregation and thermosensitive behaviors of the mixed aggregates were investigated by using 1H NMR, turbidimetry, differential scanning microcalorimetry (micro-DSC), dynamic light scattering (DLS), and fluorescence spectroscopy. The copolymer mixtures (mixed aggregates, M1 ~ M5, with different PNP-PCL content) formed nano-sized self-aggregates in aqueous environment by intra- and/or intermolecular association of hydrophobic PCL chains. The microscopic investigation of the mixed aggregates revealed that the critical aggregation concentration (cac), the partition equilibrium constants (Kv)of pyrene, and the aggregation number of PCL chain per one hydrophobic microdomain were baried by compositions of the mixed aggreagtes. Furthermore, the PNP-PCL containning mixed aggregates showed phase transition behavior, originated by coil to globule transition of PNP block upon heating, which resulted in the turbidity change, endothermic heat exchange, and reduction of particle size upon heating.