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 biotype Ⅰand Flavobacterium columnaris. Using the aqueous extracts of 6 Chinese herbal medicines, Coptis chinensis Rhem offcinale,Forsythia suspensa, Houttuynia cordata, Magnolia officinalis and Foliun 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 comeR. offcinale andF.suspensa,while H.cordata, M.officinalis and F.Isatidis do not have significant inhibitory effects.