The aim of this study was to investigate the therapeutic effect of Angelica sinensis on a rat model of diffuse interstitial pulmonary fibrosis induced by bleomycin A5. The mechanism by which A. sinensis exerts its effect is also discussed. A diffuse interstitial pulmonary fibrosis model was established in 36 male Wistar rats by an endotracheal injection of bleomycin A5 (5mg/kg). Then, these rats were randomly divided into the model group (n = 18) and the treatment group (treated with A. sinensis after modelling, n = 18). Control rats (n = 6) received an equal volume of saline. Hematoxylin-eosin (HE) staining was performed to analyse alveolitis and Masson staining, to observe pulmonary fibrosis. Collagen content was determined by hydroxyproline assay. Nuclear factor kappa B (NF-kappa B) activity was measured by electrophoretic mobility shift assay. Transforming growth factor-beta (TGF-beta) expression at mRNA level was detected by northern blotting and at protein level by enzyme-linked immunosorbent assay. The results obtained showed that the alveolitis and pulmonary fibrosis of the rats treated with A. sinensis was significantly alleviated compared with that of the rats in the model group. Treatment with A. sinensis also lowered the content of collagen, decreased NF-kappa B activity in alveolar macrophages and reduced the TGF-beta expression at the mRNA and protein level. These results indicated that A. sinensis is effective in treating and alleviating interstitial pulmonary fibrosis, possibly by lowering collagen, inhibiting the activity of NF-kappa B and reducing the TGF-beta expression.