This is the first of several articles about UV photodegradation of paper. Paper free of additives and four kinds of paper containing rosin-alum, alum only, alkylketendimer (AKD)-cation polymer, or cation polymer only were treated by UV light in air. Main wavelength of UV light (Toshiba H400-P) was 365 nm. The distance between the lamp and the sheets was adjusted to 55 cm. Photodegradation of cellulose and hemicellulose chains in paper brought about a slight increase in tensile strength and Young's modulus, and a drastic decrease in folding endurance for every paper examined so far. These findings may be explained by molecular rearrangements of degraded cellulose and hemicellulose to form new stable hydrogen bonds during irradiation. Sizing and retention chemicals have little influence on such paper properties even though some of them are chemically modified by UV irradiation. Since rosin-size has UV absorption at the relatively long wavelength region, UV treatments bring about the gradual loss of sizing effects and influence wet tensile strength and water absorption properties. Optical properties are primarily affected by additives. Rosin-sized paper showed the darkest color after UV treatment because of its UV instability. AKD seems to suppress the coloring reactions to give lower discolored paper compared with paper without additive.