Objective: To find out whether ultraviolet ray, a kind of non-ionic ray, could cause the bystander effect, the UV exposed MOLT-4 cells had been investigated. Methods: Two experiment groups were carried out, in which cells were culture and treated at two concentrations: 2 x 105/mL and 5 x 105/mL. All other treatments were the same. Part of the cells was labeled with DID and exposed to UV ray for 40 s as effect cells; other cells was untreated as bystander cells. Then, the cells were co-cultured and harvested at 4 h interval over a period of 24 h. Annexin V-FITC/PI assay was used to evaluate the bystander effect in bystander cells co-cultured with effected cells. Laser confocal microscope method was used to observe the morphologic changes of the bystander cells. Results: The percentage of cells undergoing apoptosis in the bystander cells was increased over time compared with the control group. They were 6.84%, 8.09%, 9.88%, 17.64%, 17.43%, 30.99% and 37.93% respectively in 0 h, 4 h, 8 h, 12 h, 16 h, 20 h and 24 h. When observed by laser scanning confocal microscope, the bystander cells show some classic character of apoptosis such as chromosome condense, phosphatidylserine transfer and formation of apoptotic bodies. Conclusion: Bystander effect is significant in un-irradiated bystander MOLT-4 cells when co-cultured with UV exposed cells. Objective: To find out whether ultraviolet ray, a kind of non-ionic ray, could cause the bystander effect, the UV exposed MOLT-4 cells had been investigated. Methods: Two experiment groups were carried out, in which cells were culture and treated at two concentrations: 2 x 105 / mL and 5 x 105 / mL. All other treatments were the same. Part of the cells was labeled with DID and exposed to UV ray for 40 s as effect cells; other cells was untreated as bystander cells . The cells were co-cultured and harvested at 4 h interval over a period of 24 h. Annexin V-FITC / PI assay was used to evaluate the bystander effect in bystander cells co-cultured with effected cells. Laser confocal microscope method was used to observe the morphologic changes of the bystander cells. Results: The percentage of cells undergoing apoptosis in the bystander cells was increased over time compared with the control group. They were 6.84%, 8.09%, 9.88%, 17.64%, 17.43% , 30.99% and 37.93% respectively in 0 h, 4 h, 8 h, 12 h, 16 h, 20 h and 24 h. When observed by laser scanning confocal microscope, the bystander cells show some classic character of apoptosis such as chromosome condense, phosphatidylserine transfer and formation of apoptotic bodies. Conclusion: Bystander effect is significant in un -irradiated bystander MOLT-4 cells when co-cultured with UV exposed cells.