We have investigated the structure-activity relationship between 63 natural oxycoumarin derivatives and their effects on the expression of inducible-nitric oxide synthase (iNOS) induced by lipopolysaccharide. The protein expression of iNOS was screened by Western blot analysis, and four 5,7-dimethoxycoumarins were selected as potent inhibitors of iNOS expression. In terms of structural specificity, the methoxyl group on C-5 and C-7 and the short alkyl chain (1-5 carbons) on C-6 may be essential for the potent activities. These compounds also showed inhibitory effects on nitric oxide generation and mRNA expression of inflammatory mediators, namely, iNOS and COX-2. Interestingly, the inhibitory effect on mRNA expression was specific for iNOS and was not detected for neuronal NOS. It is expected that these compounds will show anti-inflammatory activities via inhibition of the expressions of iNOS and COX-2.