Abstract: Objective To investigate the mechanism of bystander effect induced by ultraviolet irradiation.Methods The V79 cells were irradiated for 20 seconds with ultraviolet C(UVC) at 9 mJ/cm² radiation intensity.The irradiated conditioned medium(ICM) was harvested in the phases of 4,8,and 12 hours.Cell viability was evaluated by methyl thiazolyl tetrazolium(MTT) assay.The chromosome aberration was estimated with cell genetics test.Apoptosis rate of V79 cells were determined with flowcytometry(FACs).Results The cell viability increased with the prologation of harvest time of ICM.The viabilities of 4,8 and 12 hours were 61.5±3.9%,78.8±3.3%,and 84.2±4.1%,which were remarkably different from that of the control(P < 0.01).The chromosome aberration rate decreased with the rates of 12.33%,8.67%,and 5.67%.The apoptosis rate descended with the rates of 20.78±2.38%,13.45±1.99%,and 10.51±1.53%,which were noticably higher than that of the control group(P < 0.01).Conclusion The ICM harvested form the cells irradiated with 180 mJ/cm² UVC can induced bystander effect in the form of apoptosis.