Oxidative stress imposed by reactive oxygen species plays a crucial role in pathophysiology of inflammatory diseases. In the present study, sesquiterpenoids and diterpenoids isolated from Siegesbeckia pubescens, a Chinese traditional medicine used to treat arthritis, were evaluated for inhibition of NO production in activated RAW 264.7 macrophages and FMLP/CB induced O₂^- generation and elastase release in human neutrophils. In the former assay, sesquiterpenoids were more potent than diterpenoids. The C-4 carbonyl group in the carabrane-type sesquiterpenoid 3 and the C-9 ether linkage in the germacrane sesquiterpene 7 were associated with the enhanced potency. Also, for the active ent-kaurane type diterpenoids, esterification of 17-OH with isobutyric acid and acetylation of 18-OH affected the inhibition of O₂^- generation and elastase release. This report is the first to describe the inhibitory effects on oxidative stress of secondary metabolites from S. pubescens. Its findings suggest that active terpenoids from the herb could be used as lead anti-inflammatory agents.