Polygoni multiflori Ramulus (PM), the stem of Polygonum multiflorum Thunb. has been widely used as a traditional medicine for the treatment of lots of diseases. In macrophages, nitric oxide is released as an inflammatory mediator and has been proposed to be an important modulator of many pathophysiological conditions in inflammation. In the present study, it was investigated that the inhibitory effects on NO and pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6) and the mechanism of down-regulation of immune response by 85% methanol extracts of PM in mouse (C57BL/6) peritoneal macrophages. Extracts of PM (0.1, 1 mg/ml) suppressed NO production and showed inhibition of pro-inflammatory cytokines like TNF-α, IL-6 and it attenuated iNOS and COX-2 expression via down-regulation of NF-кB activation. The present results indicate that the 85% methanol extracts of PM has an inhibitory effect on the production of NO through down-regulation of iNOS expression in LPS stimulated mouse peritoneal macrophages and therefore may be beneficial in diseases which related to macrophage-mediated inflammatory disorders.