The present study was conducted to evaluate the regulation mechanism of nitric oxide(NO) by water-extracted Cyperus rotundus (WCR) in RAW 264.7 macrophages. We investigated the effects of cell proliferation in mouse spleen cell and RAW 264.7 macrophages cells. WCR enhanced mitogenic activity in the dose-response manner in mouse spleen cells and RAW 264.7 macrophages cells. In nitric oxide (NO) synthesis by WCR, WCR alone had an effect on NO synthesis. It was found that the production of NO of RAW 264.7 cells stimulated with lipopolysaccharide (LPS) could be markedly inhibited by WCR. Inhibition of NO production was achieved by reducing inducible nitric oxide syntheses(iNOS) mRNA expression. The expression of IL-I gene by WCR was investigated using reverse transcription polymerase chain reaction (RT-PCR). In RT-PCR, IL-1 family(IL-1 α, IL-1β) expressions were induced by WCR. These finding suggested that regulation of NO production by WCR may be, at least in part, associated with the regulation of iNOS mRNA expression and IL-1 family gene expression.