Abstract: In this study, six Chinese herb medicines were tested for their antibacterial activity against 4 pathogens of freshwater fish: Aeromonas sobria, A.hydrophila, Pseudomonas fluorescent biotypeⅠand Flavobacterium columnaris. Using the aqueous extracts of 6 Chinese herbal medicines, Coptis chinensis, Rhem officinale, Forsythia suspensa, Houttuynia cordata, Magnolia officinalis and Folium isatidis, we measured the diameter of the zone of inhibition by a filter paper disc test and determined the minimal inhibitory concentration by concentration gradient method. Results indicate that the aqueous extract of C.chinensis has the strongest inhibitory effect on the 4 pathogens, then come R.officinale and F.suspensa, while H.cordata, M.officinalis and F.isatidis do not have significant inhibitory effects.