Abstract: Objective To study the value of serum S100 protein in the diagnosis of radiation-induced brain injury in astrocytoma patients. Methods Serum S100 protein levels were detected by enzyme-linked immunosorbent assay in 86 astrocytoma patients in the course of radiotherapy. Results The positivity rates of serum S100 protein before, in the middle of and after radiotherapy were 0, 9.3% (8/86) and 19.8% (17/86), respectively, showing significant differences (P<0.05) between these time points for measurement. Conclusion High levels of serum S100 protein are associated with radiation-induced brain injury in astrocytoma patients and may serve as the marker for early diagnosis of the injury.