Poly(L-lactic acid)/poly(N-isopropylacrylamide) (abbreviated as LN) block copolymers were synthesized and the LN nanoparticles were prepared by simple diafiltration method. The thermal transition of the LN nanoparticles was at 32.3 degrees C, the lower critical solution temperature (LCST) of the polymer. The fluorescence spectroscopy data showed that LN was self-assembled in water to form core-shell structure nanoparticles, and the critical association concentration (CAC) value was estimated as 1.3x10(-2) g/l. From the transmission electron microscope observations, the LN nanoparticles were spherically shaped and ranged in size between 30 and 50 nm below the LCST. The hydrated size was measured by photon correlation spectroscopy, and reversible size changes were investigated by the factor of temperature. The release of indomethacin from the LN nanoparticles was thermo-sensitive due to the unique characteristic of poly(N-isopropylacrylamide).