Objective：To investigate the effects of bone marrow mesenchymal stem cells（BM-MSCs） on the pulmonary fibrosis in neonatal rats suffering from hyperoxia. Methods：BM-MSCs were isolated from male SD rats and transplanted into receptor rats. The receptor rats were devided into normal control group，normal BM-MSCs group，hyperoxia control group and hyperoxia BM-MSCs group. On the 14th day，the rats were killed and pulmonary tissue samples were taken for pathological examination and hydroxyproline level test. DNA was isolated from female pulmonary tissues and Sry gene expression was detected by reverse-transcription polymerase chain reaction（RT-PCR）. Results：Pathological examination showed significant fibrosis and structural disorder in pulmonary tissue of hyperoxia control group. The hydroxyproline level in pulmonary tissue samples was significantly higher in hyperoxia control group than in normal control group. The thickness of alveolar wall and the content of hydroxyproline in the lung were reduced significantly in hyperoxia BM-MSCs group. The results from PCR indicated that Sry gene was detected in normal BM-MSCs group and hyperoxia BM-MSCs group，however，the Sry gene expression was increased in hyperoxia BM-MSCs group compared with that in normal BM-MSCs group. Conclusion：BM-MSCs could ameliorate pulmonary fibrosis in neonatal rats suffering from hyperoxia.