Objective To study the effects of L-arginine （L-Arg） on cell proliferation, inducible nitric oxide synthase (iNOS) expression and cell cycle in human colon carcinoma cell line LS174 through nitric oxide (NO) pathway. Methods LS174 cells were cultured in medium with L-Arg at different concentrations for different times. MTT method was employed to evaluate the level of the cell proliferation. The production of NO in culture supernatants of LS174 cell was detected with enzyme reduction of nitrate. The distribution of the cell cycle was detected with the flow cytometry (FCM). The expression level of iNOS in the cells was determined by Western blot and SP immunocytochemical staining method. Results The growth of LS174 was promoted by the L-Arg at low concentration (0.125 mmol/L) and inhibited at high concentrations (0.5， 2， 8 and 32 mmol/L). The level of NO was increased with the increasing concentration of L-Arg in culture medium. To compare with the control group, the ratio of cells at S phase was increased after 48 hours’ treatments with high concentrations (0.5, 2, 8 and 32 mmol/L) of L-Arg (P＜0.05, P＜0.01); while there was no obvious difference after treatments with low concentration (0.125 mmol/L) of L-Arg (Pgt;0.05). With the increase of the concentration of L-Arg, the expression of iNOS was increased as compared with control group. The higher the concentration of L-Arg was, the better the effect. Conclusion L-Arg can induce the expression of iNOS resulting in increase the production of nitric oxide (NO). Low concentration of L-Arg can promote the growth of LS174 cells, while high concentration ones can inhibit growth and proliferation. The high concentration of L-Arg could induce S phase arrestion in the cell cycle.