Abstract: Objective To investigate the value of fractional exhaled nitric oxide （FeNO） in predicting radiation pneumonia. Methods One hundred and thirty-one thoracic tumor patients from Aug 2016 to Feb 2017 were received FeNO test before and after radiotherapy， and the relationship between FeNO of large airway （50 ml／s）， small airway （200 ml／s）， pulmonary alveolus and radiation pneumonia were analyzed. Results Of the 131 patients， the average FeNOs of large airway， small airway and pulmonary alveolus were 19.86 ppb， 9.49 ppb and 2.84 ppb before radiation. FeNO level of pulmonary alveolus increased significantly after radiotherapy（P=0.015）. After radiotherapy， grade 1， 2 and 3 radiation pneumonia were found in 43 patients， 70 patients and 18 patients. The FeNO level of large airway， small airway and pulmonary alveolus and had no relation with classification of radiation pneumonia. Conclusion FeNO of pulmonary alveolus rose significantly after radiotherapy. However， the FeNO level before and after radiotherapy cannot predict radiation pneumonia.