Abstract: Ten patients with lung cancer were carried out intraoperative radiotherapy（IORT）from April to Augest in 1993.The changes of the activity of serum guperoxide dismutase（SOD）and the content of malonaldehyde（MDA）were observed at different time after IORT. Activity of SOD rose to the peak value rapidly at the 4th hour as well as the content of MDA,it came down to a level lower than the pre-IORT value at the lst day post-radiation,with the lowest at the 3th day,SOD appeared as oxidation stress typically. The content of MDA slowed down slowlv from the peak with time went,and it was still higher than the level of pre IORT a week later,While the activity of SOD was lower than the pre-IORT level at the same time. Free radical damage could be reflected by these indexes after IORT.