Paraquat (PQ) poisoning has caused a large number of human fatalities due to the progressive and irreversible pulmonary fibrosis. Docosahexaenoic acid (DHA) is well-recognized as important modulators of multiple biological pathways that affect health and disease. A line of studies have shown that DHA supplementation is associated with the alleviation of some tissue fibrosis. In the current study, pulmonary fibrosis of rats was produced by a single oral dose of 50 mg/kg bw PQ treatment. Daily 500 mg/kg bw DHA supplementation was provided 7 days before PQ treatment and lasted for consecutive 35 days. DHA was found to ameliorate the pulmonary fibrotic alterations induced by PQ, which was evidenced by significant reduction of histological changes, hydroxyproline content and level of the transforming growth factor-β₁ (TGF-β₁) mRNA. Furthermore, the protein levels of Smad 7 and SnoN in the DHA supplemented rats were significantly increased compared with those in the rats of the PQ group. These results suggested that DHA ameliorated pulmonary fibrosis induced by PQ might be attributed to its enhancement of Smad 7 and SnoN expression.