Photodegradations of the copolymer of fluoreniridene imino methacrylate (FIMA) with methyl methacrylate (MMA) or tert-butyl methacrylate (BMA) were studied for application to photosensitive materials. Because the photoreaction of fluoreniridene imino pivalate (FIP), a model compound of FIMA polymers, yielded N-tert-butyl fluorenylidene imine and fluoreniridene imino dimer as main products, a radical cleavage reaction of the iminoxy group occurred initially. In the reaction of the FIMA copolymer in dioxane, the molecular weight decreased effectively according to the photoirradiation time. The reactions of the copolymer were studied in the film state. The main chain degradations of the BMA copolymer proceeded effectively compared with those of the MMA copolymer. The effect of temperature on the degradation was also studied using a MMA copolymer containing 2.8 mol% of the FIMA unit. The reactivity increased with increasing temperature. The maximum value obtained at 433 K had about 15 times greater sensitivity than that at room temperature.