Abstract: Objective To investigate the inhibitory effect of a tripeptide, a new arginine analog, on nitric oxide (NO) production and inducible nitric oxide synthase (iNOS). Methods Macrophages challenged with lipopolysaccharide were cultured to test the inhibitory effect of the arginine analog of different concentrations on iNOS. Primarily cultured human umbilical vein endothelial cells (HUVECs) treated with insulin were used to test the effect of the analog on endothelial NOS (eNOS). Results The new arginine analog significantly inhibited NO production in the macrophages in a dose-dependent manner, but had little effect on insulin-stimulated NO production in HUVECs. The new analog could significantly suppress iNOS activity, but had no significant inhibitory effect on constitutive NOS activity or eNOS activity. Western blot analysis showed that the new analog did not significantly affect the protein expression of iNOS. Conclusion The new analog is a NOS inhibitor with high selectivity on iNOS.