Antimicrobial activity of silk and cotton fabrics dyed with gromwell colorants was examined by shake flask method. The effects of bacterial reduction rate on dye uptake and mordanting were explored. The silk(K/S value 8) and cotton fabric (K/S value 4) dyed with gromwell colorants showed high bacterial reduction rate of 99 % and 88 %, respectively. Post-mordanted silk fabric increased at K/S value, also showed higher bacterial reduction rate than unmordanted. But the effects of antimicrobial activity on mordants showed respectively different results. Pre-mordanted cotton fabric except Sn mordant increased at K/S value, but it was not significant. Pre-mordanted cotton fabric showed significantly higher bacterial reduction rate than unmordanted. Therefore, it was considered that dye uptake of cotton fabric dyed with gromwell colorants and mordants were involved in the bacterial reduction rate. It was concluded that gromwell colorants had antimicrobial activity.