The change of degree of polymerization (DP), crystallinity and extent of oxidation during deterioration of Washi (Japanese paper) were studied. The extent of decrease of DP of cellulose in the Washi samples correlated with their acidity. The higher DP of cellulose in Washi fiber compared to that in wood fibers contributes to its permanency. It is reported that partial recrystallization of the amorphous regions in fibers during ageing results in a decrease of flexibility of the paper. From measurements of X-ray diffraction, less recrystallization of Kozo, Mitsumata and Gampi papers occurs during ageing treatment. This fact also contributes to the persistence of the strength of Washi through time. From measurements of infrared spectra of Washi samples, the amount of oxidation seems to be minimal.