The purpose of the present study was to observe the changes of expression of Fos protein using immunohistochemistry and image analysis as well as flow cytometry. The result showed that the expression of Fos protein incresed rapidly in mouse thymocytes after whole body irradiation (WBI) with 75mGy X rays. It reached the peak value at 24 h, and then gradually returned to the basic level. These findings were in accordance with our previous observation of the effect of low dose radiation on transcription level of c-fos mRNA in mouse thymocytes. The results indicate that low dose radiation could enhance the function of immune cells.