Two novel lathyrane-type diterpenoids, the Euphorbia factors L 2a (1) and L 2b (2), and their stereoisomer Euphorbia factor L 2 (3) were obtained from seeds of Euphorbia lathyris . Both Euphorbia factors L 2a and L 2b possess an unprecedented trans-gem -dimethylcyclopropane as structural feature. Also, the Euphorbia factor L 2a is the first example of a lathyrane diterpenoid with an endocyclic 12( Z )-double bond. The structures of the molecules and their absolute configurations were elucidated by comprehensive spectroscopic analyses, Cu-Kα radiation X-ray diffraction, and comparison with calculated electronic circular dichroism (ECD) data. The Euphorbia factor L 2b exhibited an inhibitory effect against U937 cell line with an IC 50 value of 0.87 μM.