Two lathyrane diterpenes (1-2) together with previous ones (3-6) were isolated from Euphorbia lathyris. The structures of 1 and 2 were established on the basis of extensive spectroscopic methods, including two-dimensional NMR techniques. Stereo configuration and conformation of 1 and 2 were analyzed by X-ray diffraction for the first time. The configuration of the unique isolathyrol diterpene 2 from the seeds of E. Lathyris, was rectified from S * to R * at C-5. All compounds were examined as modulators of multidrug resistance on MCF-7/ADM cell lines in vitro. Their structure-activity relationship was discussed from aspect of different skeletons. A class of potential P-gp inhibitors with five different types (jokinol, isolathyrol, epoxylathyrol, 7-hydroxylathyrol, lathyrol) was discovered.