Abstract： Objective To evaluate the effect of endogenous nitric oxide (NO) on the ability of 5-fluourouracil (5-FU) to induce apoptosis in the liver carcinoma Bel7402 cell line, and to observe the anti-tumor mechanism and effective adjuvant of 5-FU. Methods Cells were cultured under routine conditions with Dulbecco's modified Eagle's medium (DMEM) without L-arginine (L-Arg). We observed the expression of inducible nitric oxide synthase (Inos) and apoptosis of cells induced by 5-FU with L-Arg added to the medium. The production of nitric oxide was determined by the cell expression of Inos detected by immunohistochemical staining, and by the concentrations of nitrite and nitrate in the supernatant. Results 5-fluourouracil significantly increased the Inos expression to 0.1687±0.01968 (P<0.05, vs control group), and the concentration of nitric oxide to 213±30.2 μmol/L (P<0.05, vs control group) The apoptotic cell rate increased significantly to 17.85±0.78%, while the necrotic cell rate decreased to 32.99±0.83% (P<0.05, compared with the 5-FU group). Nω-nitro-L-Arginine methyl ester (L-NAME), the antagonist of L-Arg, can block the apoptotic effects of endogenous nitric oxide. Conclusions 5-FU had a synergistic effects with L-Arg by increasing the production of endogenous nitric oxide. Endogenous nitric oxide plays an important role in the process where 5-FU induces apoptosis in liver carcinoma cells. L-Arg may be a good adjuvant for chemotherapy with 5-FU.