Abstract: In this paper, the mechanism of action of the four Chinese herbal medicines, which are Isatis leaf, Isatis root, Forsythia fruit, Patrinia and their complex prescription on Escherichia coliinduced diarrhea of piglet, was preliminarily studied from the aspects including the bacteriostatic activity in vitro on E.coli, the antagonism against liquid retention mediated by E.coli heatlabile enterotoxin ( LT )， the antagonistic effect on diarrhea induced by castor oil, the reducing action on inflammatory exudation and the inhibiting effect on fast intestinal movement. The results showed that Isatis root, Forsythia fruit, Patrinia and their complex prescription all had more or less bacteriostasis on E.coli O149K88. Escherichia coli was cultured in bacterial culture medium which the extracts of the Chinese herbal medicines were added, and the cultural product LT was used to do experiment in ligated rabbit ileum. They all significantly or very significantly reduced the liquid retention caused by cultural supernatant fluid in ligated loops (P＜0.05，P＜0.01). When the extracts of Patrinia and LT were directly injected into ligated loops together, the liquid retention was significantly reduced (P＜0.05). Complex prescription significantly or very significantly reduced incidence and frequency of diarrhea induced by castor oil in mice (P＜0.05，P＜0.01). Isatis root very significantly inhibited the rate of mouse’s intestinal movement (P＜0.01). Patrinia, Isatis root and complex prescription also significantly or very significantly inhibited exudation of dyestuff induced by acetic acid in mouse’s abdominal cavity (P＜0.05，P＜0.01). The aforementioned results indicated that the bacteriostatic activity, the antagonism against diarrhea caused by enterotoxin LT, the antagonistic effect on diarrhea induced by medicine, the reducing action on inflammatory exudation and inhibiting effect on fast intestinal movement were important mechanism of Chinese herbal medicines on Escherichia coliinduced diarrhea of piglet. The results provided an important theoretical basis for clinical use of Chinese herbal medicines to Escherichia coliinduced diarrhea of piglet.