Nitric oxide (NO) has been reported to trigger apoptosis of a variety of cell types by P53-dependent way, which may involve in the modification of cellular radiosensitivity. The aim of this study is to investigate the effect of NO on tumor cell apoptosis induced by irradiation. Cultured Hep G2 cells were pre-incubated with 0.1 mmol/L sodium nitroprusside (SNP, a NO releaser) at 37℃in 5% CO2 for 4 h, and then Nitric oxide (NO) has been reported to trigger apoptosis of a variety of cell types by P53-dependent way, which may involve in the modification of cellular radiosensitivity. The aim of this study is to investigate the effect of NO on tumor cell apoptosis induced by irradiation. Cultured Hep G2 cells were pre-incubated with 0.1 mmol / L sodium nitroprusside (SNP, a NO releaser) at 37 ° C in 5% CO2 for 4 h, and then