The aerial part of Siegesbeckia pubescens (Compositae) has been used to treat rheumatoid arthritis and hypertension in the Oriental medicine. This crude drug has been used without process (SP-0) or with three times-process of steaming and drying (SP-3) or the nine times of that process (SP-9). To search for the antinociceptive anti-inflammatory components from this crude drug, activity-directed fractionation was performed on this crude drug. Since the CHCl3 extract was shown to have a more potent effect than other extracts, it was subjected to silica gel & ODS column chromatography to yield two diterpene compounds (1). Compound 1 was structurally identified as ent-16 (H, 17-hydroxykauran-19-oic acid, which were tentatively named siegeskaurolic acid A. A main diterpene, siegeskaurolic acid A was tested for the antiiflammatory antinociceptive effects using both hot plate- and writhing anti-nociceptive assays and carrageenan-induced anti-inflammatory assays in mice and rats. Pretreatment with siegeskaurolic acid A (20 and 30mg/kg) significantly reduced the stretching episodes, action time of mice and carrageenan-induced edema. These results support that siegeskaurolic acid is a main diterpene responsible for antinociceptive and antiiflammatory action of S. pubescens. In addition, the assays on SP-0, SP-3 and SP-9 produced the experimental results that SP-9 had more significant effects than other two crude drugs. These results suggest that the processing on the original plant may lead to the higher pharmacological effect.