Objective To observe the effect of miR-21 on bleomycin-induced pulmonary fibrosis in rats, and explore the related mechanism. Methods Peripheral blood was collected from idiopathic pulmonary fibrosis (IPF) patients (n=20) and healthy adults (n=20). Fluorescence quantitative real-time PCR was then used to measure miR-21 expression. Forty-five SD rats were randomly divided into control group, miR-21 agomir group and miR-21 antagomir group. Each group included 15 rats. After establishment of pulmonary fibrosis models by intratracheal administration with bleomycin A5, rats in control group, miR-21 agomir group and miR-21 antagomir group were injected at caudal vein with normal saline, miR21 agomir and miR21 antagomir, respectively. All rats were sacrificed on day 28 after modeling. Subsequently, the pulmonary tissues were removed for HE and Masson staining. The mRNA and protein expressions of a disintegrin-like and metalloproteinase with thrombospondin type 1 motif (ADAMTS-1), collagen type 1 (Col1) and collagen type 3 (Col3) were detected by fluorescence quantitative real-time PCR and Western blotting. Serum was separated to examine procollagen type 1 carboxyterminal propeptide (P1CP) and procollagen type 3 aminoterminal propeptide (P3NP) concentrations by ELISA. Results The level of miR-21 in peripheral blood was higher in IPF patients than in healthy adults. The alveolitis and pulmonary fibrosis extent in miR-21 agomir group was heavier than that in the control group. However, the alveolitis and pulmonary fibrosis extent in miR-21 antagomir group was improved when compared with the control group. In comparison with the control group, ADAMTS-1 mRNA and protein expression was significantly downregulated, whereas the mRNA and protein expressions of Col1 and Col3 were significantly upregulated and serum P1CP and P3NP concentrations were elevated in miR-21 agomir group. On the contrary, the level of ADAMTS-1 mRNA and protein expression in miR-21 antagomir group was higher than that in the control group; the levels of Col1 and Col3 mRNA and protein as well as serum P1CP and P3NP concentrations in miR-21 antagomir group were lower than those in the control group. Conclusion miR-21 promotes the progression of bleomycin-induced pulmonary fibrosis in rats. The mechanism is associated with downregulation of ADAMTS-1 expression and subsequent increase of pulmonary Col1 and Col3 contents.