Effect of quercetin on NO production and regulation mode of quercetin on oxidative stress, $NF{kappa}B$ activation, and iNOS expression, possible mechanisms of NO suppression in LPS-stimulated macrophages were investigated. Treatment of RAW 264.7 cells with quercetin significantly reduced lipopolysaccharide (LPS)-stimulated nitric oxide (NO) production dose-dependently ($IC_{50}$, $9.2;{mu}M$). Expression of iNOS and specific DNA binding activities of nuclear factor kB ($NF{kappa}B$) were significantly suppressed by quercetin pretreatment. Quercetin reduced thiobarbituric acid-reactive substances (TBARS) accumulation, enhancing GSH level and antioxidant activities of enzymes, such as superoxide dismutase (SOD) and catalase. These results demonstrate quercetin may ameliorate inflammatory diseases by suppressing NO production through inhibition of iNOS expression, $NF{kappa}B$ transactivation, and oxidative stress, which may be mediated partially by antioxidative effect of quercetin. Thus, quercetin appears to be used as a potential therapeutic agent for treating LPS-induced inflammatory processes.