In the present study, we screened the inhibitory effect of the extract from 50 Thai medicinal plants on an inducible-nitric oxide synthase (iNOS) expression induced by lipopolysaccharide (LPS) in mouse macrophages RAW 264.7. From this screening, the extracts of root bark of Clausena guillauminii, C. lunulata, and C. excavata (Rutaceae) were found as the extracts which showed potent inhibitory effect on the iNOS protein expression in concentration-dependent manner. Furthermore, hydrophobic active components may exist in C. guillauminii.