Euphorbia hirta L. has been widely used in India and Chinese society. The molecular pharmacology basis of its anti-inflammatory effect is revealed in this work. The ethanol extract of Euphorbia hirta L. (Eh) and its active component were studied in lipopolysaccharide (LPS)-activated macrophage cells (RAW 264.7) as an established inflammation model. After activation, nitric oxide (NO) production and expression of iNOS protein and iNOS mRNA were measured by using a colorimetric assay (Griess reagent), western blotting, and reverse transcription polymerase chain reaction (RT-PCR), respectively. The alteration in the content of PGE(2), TNFalpha, and IL-6 was concurrently monitored by ELISA. In results, we found that in the concentration range without showing cytotoxicity, Eh produced a remarkable anti-inflammatory effect via its active component of beta-amyrin and showed a dose-related inhibition of LPS-induced NO production. This phenomenon is in accordance with a substantial inhibition of iNOS protein. However, the expression of iNOS gene was unaffected by Eh treatments. Compared with indomethacin, Eh has much more potency and a specific action of NO inhibition but Eh works less specifically on PGE(2), IL-6, and TNF-alpha inhibition. The extract of Euphorbia hirta L. and its component beta-amyrin are able to block most of the iNOS protein functions and NO induction, and could therefore be new selective NO inhibitors with great potential in treating arthritis inflammation.