The purpose of this study is to investigate the dyeability, antibacterial activity on silk and rayon fabrics dyed with cochineal. The result are as follows; 1. The K/S value of silk fabric was higher than that of rayon fabric. 2. The optimum conditions are mordant concentration $0.5{sim}1%$, dyeing material concentration 2.0%(o.w.f), dyeing temperature $60^{circ}C$, PH 3 and dyeing time 30minutes. 3. Pre-mordanting method had higher dyeing absorption than post mordanting method in case of silk and rayon fabrics. 4. The antibacterial activity of dyed silk fabrics were higher than that of dyed rayon fabrics and the antibacterial activity was increased by mordanting. 5. The lightfastness and perspiration fastness of silk fabric were improved but these of rayon were not. Dyeing fastness was improved by Fe mordanting on both fabrics.