Photoreactive 3,4-diacetoxycinnamic acid- (DACA) terminally conjugated poly(L-lactide) (PLLA) (DACA-PLLA) nanoparticles with different diameters were prepared by mixing acetonitrile as a good solvent and water as a poor solvent. The average diameters and morphologies of the nanoparticles prepared under various conditions were evaluated by dynamic light scattering (DLS) measurement and scanning electron microscopic (SEM) observation. The particle size was controllable from about 100 to 300 nm by altering the composition ratio of the mixed solvent, the concentration of the polymers, or the L-lactic acid (LLA) unit number. The particle size increased upon decreasing the LLA unit number or increasing the polymer concentration and water content in the mixed solution. Interestingly, the DACA-PLLA nanoparticles showed different size change properties following UV irradiation at lambda > 280 nm before or after particle formation. When UV irradiation was performed after particle formation, only a few percent (ca. 3%) size decrement was observed, although a significant size decrement (ca. 30%) was detected after UV irradiation before particle formation. The DACA-PLLA nanoparticles showed weight decrement during accelerated hydrolysis experiments (pH = 10.0, 37 degrees C) for 5 days, whereas the particle diameters were unchanged completely. DACA-PLLA nanoparticles have great potential as a photoreactive PLLA nanoparticles in the various applications.