Abstract: Objective To investigate the effects of pentoxifylline(PTX) on radiation induced-cell cycle redistribution and radiasensitivity of human hepatocellular carcinoma cell line Hep3b.Methods MTT assay was performed to evaluate the cytotoxicity of PTX on p53-defective human hepatocellular carcinoma cell line Hep3b and clonogenic assay employed to observe its effects on the radiosensitivity of the cells quantified by calculating the sensitive enhancement ratio(SER).Flow cytometry was performed to observe the cell cycle changes of Hep3b cells in response to X-ray irradiation and the interventional effect of PTX.Results The cytotoxicity of PTX on the cells increased in a dose-dependent manner following a 48-hour treatment,with the optimal dose range of 1-5 mmol/L.A sub-toxic dose of PTX at 2 mmol/L was then used in subsequent experiments.Clonogenic survival assays up to 12 Gy demonstrated that p53-defective Hep3b cells(SER of 2.68±0.24) were sensitized by PTX(2 mmol/L).PTX(2 mmol/L) treatment following exposure to irradiation(6 Gy) resulted in abrogation of radiation-induced G2/M arrest of Hep3b cells,and the proportions of Hep3b cells in G2/M phase were 86.8% and 14.8% after exposure to 6 Gy alone and 6 Gy plus 2 mmol/L PTX,respectively.Conclusion Radiosensitization by PTX is possibly associated with the abrogation of G2/M arrest in Hep3b cells following radiation exposure,suggesting that potential clinical application of PTX may enhance the efficacy of radiotherapy against hepatocellular carcinoma.