The reaction-induced deformation and its recovery process in the nanometer scale of a photo-cross-linkable poly(ethyl acrylate) were in situ studied by using Mach–Zehnder interferometry. The ON–OFF modulated irradiation using two different UV wavelengths, 365 nm and 297 nm, was carried out to elucidate the effects of the reversible cross-link on the deformation kinetics of the polymer. It was found that the irradiation time-dependence of the crosslink density generated during the ON–OFF irradiation process deviates from the data obtained by continuous irradiation process, revealing the relaxation effect of the forming poly(ethyl acrylate) networks in the dark during each irradiation cycle. Furthermore, the recovery of the sample thickness can be achieved by photodissociation of the anthracene photodimers by irradiating the cross-linked sample with 297 nm UV light. These experimental results reveal the possibility of reversible control the shrinkage and swelling proceses of a polymeric film by using the photodimerization of anthracene.