OBJECTIVE： To investigate the effects of Buyang huanwu decoction on endothelial-mesenchymal transformation （EndMT） of lung tissue in idiopathic pulmonary fibrosis （IPF） model rats， and to explore its potential mechanism. METHODS： Male SD rats were randomly divided into normal group， model gorup， dexamethasone group [0.405 mg/（kg·d）]， Buyang huanwu decoction low-dose， medium-dose and high-dose groups [6.435， 12.87， 25.74 g/（kg·d）， by raw material]， with 8 rats in each group. Except for normal group， other groups were given endotracheal injection of bleomycin to induce IPF model. On the second day after modeling， normal group and model group were given water intrgastrically [10 mL/（kg·d）]； administration groups were given relevant medicine intragastrically， once a day， for consecutive 28 days. 24 h after last medication， the expression of endothelial cell markers [platelet endothelial cell adhesion molecule 1， vascular endothelial cell cadherin] and interstitial cell markers [α-smooth muscle actin， fibroblast specific protein 1] were detected by immunohistochemistry method. The expression of Notch4 and DLL4 in lung tissue of rats were detected by Western blotting assay. RESULTS： Compared with normal group， the expression of endothelial cell markers were decreased significantly in lung tissue of model group， while the expressipon of interstitial cell markers， Notch4 and DLL4 were increased significantly （P＜0.01）. Compared with model group， the expression of endothelial cell markers in lung tissue of rats were increased significantly in administration groups， while Buyang huanwu decoction low-dose group was significantly lower than dexamethasone group； the expression of interstitial cell markers， Notch4 and DLL4 were decreased significantly， while Buyang huanwu decoction low-dose group was significantly higher than dexamethasone group （P＜0.05 or P＜0.01）. CONCLUSIONS： Buyang huanwu decoction can relieve IPF of model rats by intervening in EndMT， the mechanism of which may be associated with inhibiting DLL4/Notch4 singaling pathway.