Objective To study effects of bone marrow mesenchymal stem cells on pulmonary silicosis, and to explore its mechanism. Methods BMSCs were harvested from 3-6 week old male SD rats. Fifty-four healthy female SD rats were randomly divided into saline control group, model group and BMSCs transplantation group, 18 in each group. The rats were sacrificed on day 1, 3, 7, 14, 28 and 56 after pulmonary silicosis was induced in the animals. The morphology changes and collagen deposition of lung tissues were investigated by HE staining and Masson staining. The expression of sex-determining gene (sry) and surfactant protein-c (SP-C) were detected by double-immunofluorescence labeling. Western blotting estimated the expression of TNF-α and MMP-9 in lung tissues. Results HE staining and Masson staining showed that alveolitis, pulmonary fibrosis and collagen area of lung tissue in BMSCs transplantation group significantly decreased as compared with that of the control and model groups. Sry positive cells were observed in the lungs tissue from BMSCs transplantation group, and some of sry positive cells also expressed SP-C, the marker of type II pneumocytes. The level of TNF-α and MMP-9 in the model group was significantly higher than that in the control group(P ＜0.05), and had different characteristics at different time points. At the corresponding time, the level of TNF-α and MMP-9 in BMSCs transplantation group was significantly lower than that in the model group(P＜0.05)，higher than that in the control group (P＜0.05). Conclusion BMSCs differentiate into type II pneumocytes and downregulate the expression of TNF-α and MMP-9, which may explain why BMSCs transplantation improves the SiO₂ induced fibrosis.