Abstract: Objective To explore whether Cetuximab(C225)can enhance the radiosensitization to human pulmonary adenocarcinoma A549 cells line in vitro or to initially disclosure its mechanism.Methods A549 cells were cultured in vitro and treated with different concentrations of C225.Cell Counting Kit-8(CCK8)assay was utilized to measure the inhibition of cell proliferation and determine 50% inhibition concentration(IC₅₀), and then 1/5 IC₅₀ was selected as the concentration in the follow-up experiments.The radiosensitizing effect of C225 on A549 cells was investigated by clonogenic assay.The cell survival curve was fitted by single-hit multi-target model and sensitizing enhance rate(SER)was calculated.Apoptosis and cell cycle distribution of A549 cells were measured by flow-cytometry(FCM)analysis.Results The proliferation of A549 cells was inhibited which showed an obvious dose-effect relationship, and the IC₅₀ of 48h was 18.24μg/mL.The group of C225 plus radiation displayed the values of SF₂, D₀, D_q, which were significantly lower than those of radiation alone group(P＜0.05), and the sensitizing enhance rate(SER)of D₀ was 1.40.C225 plus radiation significantly enhanced the radiation-induced apoptosis in A549 cells(P＜0.05).The cell cycle analysis showed that cells arrested in G₀/G₁ phase by C225 treatment(P＜0.05).The number of cells in G₂/M phase was increased in radiation group(P＜0.05), and cells in C225 plus radiation group were arrested both in G₀/G₁ and G₂/M phrases(P＜0.05).The number of cells in S phrase of three groups was decreased, compared with control group(P＜0.05).Conclusion C225 could enhance the radiosensitivity of A549 cells line, which is probably associated with the inhibition of proliferation and sub-lethal damage repair of tumor cells.The enhancement of radiation-induced apoptosis and cell cycle was arrested in G₀/G₁ phase.