In order to investigate the effect of retinoic acid (RA) on the pathology and Smad3 expression in the pulmonary injury induced by radiation, wistar rats were irradiated with 6MV-X rays linear accelaretor at dose of 15Gy and RA was used as interfering agent by setting control group(group A), treatment group(group B), radiation group(group C) and radiation with RA treatment group(group D). In 1, 2, 4, 8 weeks after irradiation, HE staining and Massion staining were used to detect the pathological change of pulmonary tissues while immunohisto-chemistry staining was used to measure the expression of Smad3 level. The study shows that inflammatory cells exudation occurs in alveolar spaces at one week after irradiation then followed by interstitial edema, shrinkage and destroying of structure. The local consol and collagen fibers appear in pulmonary interstitial at 4 and 8 weeks after irradiation. No significant difference of pathology has been observed between the simple treatment group and the control group, but obviously decrement has been observed in group C and group D. The expression of Smad3 has been enhanced significantly in group C and group D at 1, 2, 4, 8 weeks(,p,<,0.00001)after irradiation compared with that of group A while it is weakened in group D compared with group C, especially at 4, 8 weeks after irradiation. However, no statistical difference has been found between group A and group B. Accordingly, Smad3 does participate in the process of radiation induced lung injury while RA restrains the expression of Smad3 in protein level. And it may provide a useful evaluation to curing of radiation- induced lung injury.