【Abstract】 Objective To investigate the effect of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, on the growth of colorectal cancer xenografts in vivo and on tumor-associated neovascularization. Methods Twenty BALB/c nude mice were randomly divided into control group and study group equally. Human colorectal cancer cell line SL174T was inoculated subcutaneously into nude mice to form transplantation tumors. Saline 0.2 ml was intragastric-administrated to mice in control group and L-NAME （4 mg toties guoties) was administrated orally to mice in study group three times per week for four weeks. The changes of tumors in both groups were recorded and the microvessel density (MVD) was also measured by immunohistochemistry assay. Results L-NAME significantly inhibited the growth of colorectal cancer xenografts in nude mice. The weight of transplantation tumor reduced with the inhibitory rate of 41.36%, and the inhibitory rate of tumor volume was 43.48% in study group. MVD in the study group and control group were 14.83±2.10 and 21.04±3.11, respectively, which showed that the former was significantly lower than that of the control group （P＜0.05）. Conclusion L-NAME may inhibit the growth of colorectal cancer via the suppression of tumor neovascularization.