Objective To explore the treatment effect of bone marrow mesenchymal stem cells( BMSCs)transplantation in ratmodel of bleomycin-induced pulmonary fibrosis. Methods BMSCs fromten-day-old SDmale rat were cultured and marked with 4, 6-diamidino-2-phenylindole( DAPI) . Seventy female SD rats were randomly divided into four groups. Group A( n = 21) was intratracheally injected with saline as control. Group B( n = 21)were intratracheally injected with BLMA5 to establish pulmonary fibrosis. Group C( n = 21) was injected with BLMA5 intratracheally and BMSCs intravenously via tail vein simultaneously. Group D( n = 7) was injected with BMSCs 14 days after BLMA5 injection. The rats were sacrificed on 7th, 14th and 28th day respectively( rats of group D were on28th) . HE and Masson stainings were performed to observe lung pathological changes. Fluorocyte marked with DAPI was analyzed by fluorescent microscope. Sex determining region Y( SRY) gene were detected by PCR. The lung levels of HYP, tumor necrosis factor-α( TNF-α) and transforming growth factor-β1 ( TGF-β1 ) were measured by ELISA. Results ( 1) In group C and D, BMSCs marked with DAPI were detected in lung frozen section on 7th, 14th and 28th day, and SRY gene of male rats were detected by PCR. ( 2) Alveolitis was most obvious on 7th day and pulmonary fibrosis was most severe on 28th day in group B compared to other three groups( P lt;0. 05 or 0. 01) . Alveolitis and pulmonary fibrosis in group C and D were significantly alleviated compared to group B( P lt; 0. 05) , but still more severe than group A( P lt; 0. 05 or 0. 01) , which in group D was more severe compared to group C( P lt;0. 05) . ( 3) HYP level in group B, coincided with fibrosis, began to increase on7th day and reached the peak on 28th day, significantly higher than other three groups( P lt;0.05 or 0. 01) . TNF-αlevel in group B was highest on 7th day, then descended, which was significantly higher than group A and C on 14th day and not obviously different from other three groups on 28th day. TGF-β1 level in group B was highest on 28th day which was different significantly fromother three groups. Conclusion BMSCs can colonize in the recipient lung tissue and effectively prevent the development of pulmonary fibrosis of rats induced by BLMA5, especially in the early stage.