The radiosensitizing effects of luteolin were studied in the gastric cancer cell line SGC-7901. SGC-7901 cells were treated with luteolin or/and irradiation, and radiosensitizing effects were assessed by colony-forming assay with cells and nude mice. In order to study the underlying mechanism, the levels of apoptosis-related proteins, the activities of caspase-3 and -9, and the production of PGE2 were measured. The results showed that luteolin could enhance irradiation-induced colonogenic inhibition and the activities of Caspase-3 and -9. The remarkable down-regulation of Bcl-2 and release of cytochrome C were also observed. In addition, significantly reduced production of PGE(2) was observed in luteolin plus radiation treatment by ELISA, as well as decreased expression levels of VEGF and HIF-1 alpha. Finally, luteolin significantly enhances the radioresponse of human tumors transplanted into nude mice. Our results indicate that luteolin may be a promising radiosensitizer for use in the treatment of gastric cancer.