A new clerodane diterpene, 2β-methoxyhardwickiic acid (1), and four known compounds (2–5) were isolated from the twigs of Monoon membranifolium. The structure of the new compound was determined by extensive spectroscopic methods and ESITOFMS data. The absolute configuration of 1 was established by a comparison of its ECD spectrum and specific rotation with those of related previously reported compounds. All compounds were evaluated for their nitric oxide (NO) production inhibitory activities in RAW264.7 macrophage cells. Compounds 3 and 5 inhibited NO production with IC50 values of 16.1 and 28.9 μM, respectively, which were better than that of standard compound, indomethacin (IC50 = 32.2 μM).