Objective To study the radiosensitizing effect of nano-gold nano-silver particles in hepatocellular carcinoma cells (HepG2) in vitro and the possible mechanisms.Methods MTT assay and clonogenic assay were performed to determine the killing effect of nano-gold and nano-silver particles in HepG2 cells.Flow-cytometry was used to measure cell apoptosis and cell cycle distribution.Western blotting was used to measure the expression of Caspase-3,Bax and Bcl-2.ELIASA was used to determine the content of catalase (CAT),superoxide dismutase (SOD),and total glutathione (GSH).Results Nano-gold and nano-silver particles inhibited the proliferation of HepG2 cells with IC50 of 6.51 μg/ml and 2.47 μg/ml,respectively.Nano-gold and nano-silver particles significantly enhanced the radiosensitivity of HepG2 cells.Obtained by Dq,the SER of 1/5 IC50 nano-gold and nano-silver particles were 1.37 and 1.48,and 1/10IC50 with 1.11 and 1.09.Nano-gold and nano-silver particles increased the expression of Caspase-3 and Bax and reduced the exprcssion of Bcl-2.CAT,SOD and total GSH were significantly reduced.Conclusions Nano-gold and nano-silver particles can enhance the radiation sensitivity of HepG2 cells.Specific sensitizing mechanism may be the activation of the mitochondrial apoptosis pathway and the induction of reactive oxygen species in apoptotic pathways,which ultimately induces apoptosis.