This study was made to investigate the effects of mimosa tannin and tannic acid on change of properties and photodegradation of silk fabrics according to tannin concentration and irradiation time. The results were as follows. 1. As concentration of tannin increased, thickness, fabric count and stiffness showed no difference. But tensile strength were increased, and elongation were decreased. 2. When concentration of tannin is high, tannin adhered to surface of fabrics. 3. fR absorption bands of untreated silk fabric appeared at $3400cm^{-1},; 1640cm^{-1},;1445cm^{-1},;1235cm^{-1},;and;675cm^{-1}$, but those of silk fabric treated with tannic acid appeared at $1710cm^{-1};and;3400cm^{-1}$ by treatment of mimosa tannin, respectively. IR absorption bands of photoirradiated silk fabrics appeared at $3400cm^{-1},;3280cm^{-1},;2960cm^{-1},;2920cm^{-1},; 1720cm^{ -1},;1380cm^{-1},;1280cm^{-1},;1120cm^{-1},;and;1070cm^{-1}$. But those bands were decreased at the silk fabrics treated with mimosa tannin and tannic acid.