Abstract: Objective To determine the role of the heterocyclic compound 8-oxo-3-thiomorpholino-8H-acenaphtho1,2-b pyrrole-9-carbonitrile (S1) pyrrole-9-carbonitrile (S1) in sensitizing of human prostate cancer cell line PC3 to radiation. Methods Flow cytometry analysis was used to detect the apoptosis ratio of PC3 cells after treated with different dose of X-ray irradiation or different concentration of S1 or combination of them; Bcl-2 protein levels in PC3 cells treated with S1 in different time point was detected by Western blot method. Results The results showed that X-ray could induce apoptosis of PC3 cells with a dose-dependent manner. There was not significant changes in apoptosis of PC3 cells after 2Gy or 5Gy X-ray irradiation, however, there was obviously apoptosis after 10Gy or 20Gy X-ray irradiation (P<0.01). It was also demonstrated that S1 could induce apoptosis ratio change in PC3 cells with a dose-dependent manner. The ratio of apoptosis percentage in 5μmol/L group and 10μmol/L group was significantly higher than that of control group (P<0.01). Bcl-2 protein level was decreased gradually in PC-3 cells treated with S1 for 2 to12h. After treated with S1, PC3 cells were exposed to 2Gy or 8Gy X-ray irradiation, the ratio of apoptosis was markedly elevated compared with irradiation alone group or S1 alone group (P<0.01). Conclusion Our results incated that the small compound S1 could sensitize PC3 cell line to X-rays irradiation. It's a potential new therapeutic method to improve the therapeutic efficiency of human prostate cancers which can decrease radiation-induced lesion.