Radiation-induced pulmonary fibrosis is a common disease and has a poor prognosis owing to the progressive breakdown of gas exchange regions in the lung. Recently, a novel strategy of administering mesenchymal stem cells for pulmonary fibrosis has achieved high therapeutic efficacy. In the present study, we attempted to use human adipose tissue-derived mesenchymal stem cells to prevent disease in Sprague-Dawley rats that received semi-thoracic irradiation (15 Gy). To investigate the specific roles of mesenchymal stem cells in ameliorating radiation-induced pulmonary fibrosis, we treated control groups of irradiated rats with human skin fibroblasts or phosphate-buffered saline. After mesenchymal stem cells were infused, host secretions of hepatocyte growth factor (HGF) and prostaglandin E2 (PGE2) were elevated compared with those of the controls. In contrast, tumour necrosis factor-alpha (TNF-α) and transforming growth factor-beta1 (TGF-β1) levels were decreased after infusion of mesenchymal stem cells. Consequently, the architecture of the irradiated lungs was preserved without marked activation of fibroblasts or collagen deposition within the injured sites. Moreover, mesenchymal stem cells were able to prevent the irradiated type II alveolar epithelial cells from undergoing epithelial-mesenchymal transition. Collectively, these data confirmed that mesenchymal stem cells have the potential to limit pulmonary fibrosis after exposure to ionising irradiation.