Reports studies on the mechanisms of the changes in physical properties caused by UV treatments of paper. Degree of polymerization, UV absorptive spectrum, infrared (IR) spectrum, and NMR (¹H & ¹³C) were measured. Cellulose in amorphous regions and hemicellulose are depolymerized through oxidative and/or radical reactions by UV treatments. This depolymerization leads to the formation of a stable intermolecular hydrogen bond by molecular rearrangement, and thus the decrease in hydroxyl groups, which can interact with moisture or water, and causes the decrease in hydrophilic properties of paper. This new hydrogen bond formation brings about the increase in Young's modulus, and even the slight decrease in moisture content of the paper probably causes the drastic decrease in double folds. On the other hand, porous structures of paper were found to be unchanged by UV treatments, and cross-linking reactions between cellulose chains were not likely to form during the treatments. Rosin chemicals are degraded to form hydrophilic groups such as carboxylic acid from their double bonds by UV treatments. This causes the decrease in sizing strength of rosin-sized paper, while the UV treatments bring about an increase in paper strength for rosin-free paper, such as neutral paper, owing to the formation of the stable hydrogen bond, as described above.