The main eicosanoids inflammatory mediators, prostaglandins and leukotrienes, are both generated from arachidonic acid (AA; 20:4 n-6). AA is a member of polyunsaturated fatty acids (PUFAs). Numerous studies have demonstrated that various contents of PUFAs can modulate the inflammatory responses. However, fewer studies have examined n-9PUFAs and their effects on the inflammatory responses. In the present study, the role of 5,8,11-cis-eicosatrienoic acid (ETrA; 20:3 n-9, also called Mead acid) in the inflammatory responses has been investigated. The anti-inflammatory activities of ETrA were examined using an in vitro macrophage system and the inhibitory effect was confirmed by western blot analysis for iNOS and COX-2 expressions. The interactions between ETrA and COX-2 protein were simulated to produce a computer modeling protein-ligand complexes and the results suggest a possible mechanism for the effects of ETrA. In this study, we described a significant inhibition of the inflammatory activities initiated by ETrA. Since ETrA is a substance presented in the tissues of young animals, we therefore anticipate that ETrA can be utilized as a natural therapeutic supplement to inhibit inflammatory activities.