To develop an intravenous delivery system of all-trans retinoic acid (ATRA) for the cancer therapy, poly(L-lactic acid) nanoparticles were prepared and characterized. Emulsification-solvent evaporation method was chosen to prepare submicron sized nanoparticles. Spherical nanoparticles less than 200 nm in diameter with narrow size distribution were prepared, and the entrapment efficiency of drug was more than 95%. The endothermic peak at $183^{circ}C$ and X-ray crystallographic peak of ATRA appeared in the nanoparticle system, suggesting the inhibition of crystallization of ATRA by polymer adsorption during the precipitation process. ATRA was released at $37^{circ}C$ for 60 days and the release rate was dependent on the concentration of drug incorporated in the nanoparticles. While ATRA was unstable in the light, it was very stable at $4^{circ}C$. These results suggest the usefulness of PLA nanoparticles as a sustained and prolonged release carrier for ATRA.