AbstractThe relationship between the activity to initiate graft copolymerization under photo‐irradiation and the photo‐induced radicals of periodic acid‐oxidized cellulose (aldehyde cellulose) was investigated. Aldehyde cellulose proved to have a high activity to initiate graft copolymerization under photo‐irradiation, and the effect was profound, especially for hydrophilic vinyl monomers such as acrylic acid and acrylamide. By studying the ESR spectrum of photo‐irradiated aldehyde cellulose, the formation of a radical giving a singlet spectrum with linewidth of 14–15 G and agvalue of 2.001 was observed. This was assigned to an acyl radical orginating in the aldehyde group of the sample. Employing low molecular weight aldehydes, it was confirmed that an acyl radical formed on aldehyde compounds by photo‐irradiation has a function sufficient to initiate the graft copolymerization of vinyl monomers. It was concluded that the high activity needed to induce the graft copolymerization of aldehyde cellulose under photo‐irradiation was based on an acyl radical which originated in the aldehyde grou