Abstract: Objective To explore the radiosensitivity effect and mechanism of action of Arsenic trioxide(Arsenic trioxide ，As₂O₃) on esophageal carcinoma EC-1 cell line.MethodThe radiosensitivity effect of Arsenic trioxide(As₂O₃) on esophageal carcinoma EC-1 cell line by fitting the irradiation dose-survival curve using ‘multitarget click mathematical model'.Flow cytometry(FCM) was used to detect the changes of cell apoptosis and cell cycle. Results As₂O₃ had an obvious radiosensitivity effect on esophageal carcinoma EC-1 cells.After treatment with 3 μmol／L As₂O₃ for 48 hours，the sensitization enhancement ratio(SER) is 1.285.The G₂/M phase ratio is increased while the G₀/G₁ phase and the S phase ratio decreased after treatment.The apoptotic index was significantly increased(P<0.05),compared with the control group. Conclusion As₂O₃ has an obvious radiosensitivity effect on esophageal carcinoma EC-1 cells，and the mechanism of action may be related to decreased repair capacity of the EC-1 cells,the cell cycle arresting is G₂/M phase,the decreased cell number of G₀,G₁ and S phase and increased apoptosis.