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