We established a polymerization condition of 3-hydroxycinnamic acid, 3HCA, having photoreactive units over all the main chains. The polymers showed two types of photoreaction, that is, intramolecular E-Z isomerization and interchain [2 + 2] cycloaddition. Spectroscopic studies showed that the rate of the isomerization was higher than that of the cycloaddition. Although the original fiber was very brittle, the short-time UV-irradiation made the fiber softer and more elastic because the chain bending by the isomerization preferentially occurred. On the other hand, the long-time irradiation made the fiber harder because of the cross-linking by the cycloaddition, and simultaneously enhanced the elasticity. Besides we found that the cast film of P3HCA showed a photo-bending behavior with the convex surface facing the UV-lamp by the initial E-Z isomerization. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1112-1118, 2011