As the basic study to investigate the mechanism of improvement in the dyeing fastness of natural dyes on silk fabrics the optical behavior of tannin solution, the optimal condition of tannin treatment and the changes of the physical structure of silk fabrics were studied in the treatment of tannin. Results obtained were as follows ; 1 The K/S values of tannin-treated silk fabrics were not changed by tannin content on silk fabrics but yellowing of tannin in treatment condition. 2. As the concentration of tannin was increased in its treating condition, the tannin content and K/S vague of tannin-treated silk fabrics were increased. 3. Because tannin gave rise to yellowing by hydrolysis and oxidation at high temperature, the higher tannin-treating temperature was, the lower the tannin content and K/S values of silk fabrics 4. The tannin content of silk fabric was higher in acidic treating condition and the highest K/S value was showed at pH 5. 5. As the tannin content of silk fabric was increased, the value of $xi$ -potential of it became negative. 6. As the tannin content of silk fabric was increased, the crystallinity and crystallite size of it were decreased. This meant that the fraction of amorphous region in the fibers increased. And the degree of orientation of it was decreased with the tannin treatment.