Six new lathyrane diterpenoids (1-6) and 10 known analogues (7-16), were separated from the seeds of Euphorbia lathyris. The absolute configuration of 1 was determined by X-ray crystallography, and the C-2' configuration of 5 was elucidated by comparing experimental and calculated ECD data. These compounds were studied for their inhibition against nitric oxide (NO) generation induced by lipopolysaccharide in RAW264.7 macrophage cells. Compounds 1-3, 7, 9, 11, 13, 14, and 16 displayed inhibitory effects on NO production, with IC50 values of 2.6-26.0 μM. The new compound 1 (IC50 3.0 ± 1.1 μM), with no obvious cytotoxicity, was selected for further experiments. The production of cytokines such as IL-6 and IL-1β, as well as the protein expression of iNOS, NF-κB, and phosphorylated IκBα, was reduced by 1 dose-dependently. These results suggested that lathyrane diterpenoids may be used as potential anti-inflammatory agents and are worth being further researched.