Abstract: Studies have shown that signal transducer and activator of transcription 3（STAT3） is over-expressed in many malignant tumors. This signal pathway is closely related to the proliferation， differentiation and apoptosis in tumor cells. The activation of the STAT3 pathway promotes tumor growth and invasion. Emerging studies demonstrated that the abnormal activation of STAT3 resulted in the decrease of tumor radiosensitivity and avoidance of being damaged by radiation. The expression of STAT3 in tumor can be used in evaluating curative effect and prediction of prognosis. Inhibition of STAT3 pathway can markedly reverse the radiotherapy tolerance. The continue research on STAT3 can provide us with more tumor targeting strategies.