Objective : Many traditional herbal remedies exhibit several beneficial effects including anti-inflammation. Euonymus alatus (Thunb.) Sieb (EA), known as Gui jun woo in Korea, has long been used in folk medicine to regulate Qi (bodily energy) and blood circulation, relieve pain, eliminate stagnant blood, and treat dysmenorrhea in oriental countries. The exact mechanism of the anti-inflammatory action of Euonymus alatus (Thunb.) Sieb (EA), however, has not been determined. Methods: Since there is increasing evidence that nitric oxide (NO) plays a crucial role in the pathogenesis of inflammatory diseases, this study was undertaken to address whether the methanol (MeOH) extract and its fractions of the bark of EA could modulate the expression of inducible NO synthase (iNOS) in thioglycollate-elicited murine peritoneal macrophages and murine macrophage cell line, RA W264.7 cells. Results: Stimulation of the peritoneal macrophages and RAW264.7 cells with $interferon-gamma;(IFN-gamma)$ and lipopolysaccharide (LPS) resulted in increased production of NO in the medium. However, the butanol (BuOH) fraction of the MeOH extract of EA barks showed marked inhibition of NO synthesis in a dose-dependent manner. The inhibition of NO synthesis was reflected in the decreased amount of iNOS protein, as determined by Western blotting. The BuOH fraction did not affect the viability of RA W264.7 cells, as assessed by methylthiazol-2-yl-2, 5-diphenyl tetrazolium bromide (MTT) assay; rather, it reduced endogenous NO-induced apoptotic cell death via inhibition of NO synthesis in RAW264.7 cells. On the other hand, the MeOH and BuOH fraction showed no inhibitory effect on the synthesis of NO by RAW264.7 cells, when iNOS was already expressed by the stimulation with $IFN-gamma$ and LPS. Conclusion: Collectively, these results demonstrate that the MeOH and BuOH fraction inhibits NO synthesis by inhibition of the induction of iNOS in murine macrophages.