Mechanical and optical properties of paper prepared from a mixture of bleached kraft pulp and high-yield pulp (GP) were examined in order to elucidate the influence of lignin on paper exposed to UV irradiation. Sheet density, tensile strength, Young's modulus, and double folds decrease with increase in GP content. The Kubelka-Munk equation was found to be applicable to the investigation of optical properties such as printing opacities, scattering coefficients, and absorption coefficients of those sheets with different mixing ratios. Changes in tensile strength, Young's modulus, double folds, wet tensile strength, water absorption, brightness, and K/S values of those sheets treated by UV light were studied. The strength of double folds was found to decrease drastically by UV treatments, although GP content, i.e., the lignin content of sheets, does not accelerate the decrease in double folds. On the other hand, GP content has a strong influence on discoloration of the sheets treated by UV light. The rate constant of discoloration by UV light increases linearly up to 15% of GP content. Research showed that the decrease in double folds stemmed from depolymerization of cellulose chains by UV light. Thus, the results obtained in this work indicate that reactive groups formed from lignin by UV irradiation in the presence of oxygen, such as radicals, peroxides, singlet molecular oxygen, superoxide anions, etc., do not take part in depolymerization reactions of cellulose.