Reversible size changes in degradable nanoparticles composed of bio-based cinnamate derivative copolymers, poly(3,4-dihydroxycinnamic acid-co-4-hydroxycinnamic acid) [P(3,4DHCA-co-4HCA)], were observed for the first time. The 860 nm diameter decreased to almost one-half (420 nm) during UV irradiation over 280 nm and rapidly recovered to 620 nm upon subsequent irradiation at 254 nm. Results from UV-vis and H-1 NMR spectra suggested that diameter-changing phenomena corresponded to [2 + 2] cycloaddition formation and cleavage of the cinnamate groups. This significant diameter change was reproduced for at least three cycles, and the reason for this significant size change seemed to be due to the fact that all units of the copolymer contained photochromic groups. Furthermore, these photo-cross-linked bio-based nanoparticles showed various size change behaviors during hydrolytic degradation depending on the degree of cross-linking. Photosensitive degradable nanoparticles may be useful as novel, size-controllable carriers for environmental and biomedical fields.