Euphorbia lathyris (Caper spurge) is a toxic and potent Chinese materia medica (T/PCMM).This study sought a method for identifying five diterpenoids (Euphorbia factors L 1 L 3,L 7a and L 8) with the spectra of UV and mass,quantifying three diterpenoids L 1,L 2,and L 8 in crude extracts of unprocessed and processed E.lathyris seeds by liquid chromatography/electrospray ionization mass spectrometry (LC-ESI-MS).The analysis was achieved on an Agilent Eclipse XDB-C18 column (4.6 mm 150 mm i.d.,5 mm) with an isocratic elution with a mobile phase consisting of water and acetonitrile at a flow rate of 0.25 mL/min at column temperature of 30 1C and UV detection was set at 272 nm.An ESI source was used with a positive ionization mode.The calibration curve was linear in the ranges of 9.9-79 mg/mL for Euphorbia factor L 1,3.8-30.5 mg/mL for Euphorbia factor L 2,and 1.0-20.6 mg/mL for Euphorbia factor L 8.The average recoveries (n 6) of three diterpenoids were 98.39%,91.10% and 96.94%,respectively,with RSD of 2.5%,2.4% and 2.1%,respectively.The contents of the three diterpenoids in processed E.lathyris seeds were 3.435,1.367 and 0.286mg/g,respectively,which decreased more sharply than those in unprocessed E.lathyris seeds which were 4.915,1.944 and 0.425mg/g,respectively.The method is simple,accurate,reliable and reproducible,and it can be applied to control the quality of unprocessed and processed E.lathyris seeds. Euphorbia lathyris (Caper spurge) is a toxic and potent Chinese materia medica (T / PCMM). This study sought a method for identifying five diterpenoids (Euphorbia factors L 1 L 3, L 7a and L 8) with the spectra of UV and mass , quantifying three diterpenoids L 1, L 2, and L 8 in crude extracts of unprocessed and processed E. lathyris seeds by liquid chromatography / electrospray ionization mass spectrometry (LC-ESI-MS). The analysis was achieved on an Agilent Eclipse XDB- C18 column (4.6 mm 150 mm id, 5 mm) with an isocratic elution with a mobile phase consisting of water and acetonitrile at a flow rate of 0.25 mL / min at column temperature of 30 1C and UV detection was set at 272 nm. An ESI source was used with a positive ionization mode. The calibration curve was linear in the ranges of 9.9-79 mg / mL for Euphorbia factor L 1, 3.8-30.5 mg / mL for Euphorbia factor L 2, and 1.0-20.6 mg / mL for Euphorbia factor L 8. The average recoveries (n 6) of three diterpenoids were 98.39%, 91.10% and 96.94%, respectively , with RSD of 2.5%, 2.4% and 2.1%, respectively. These contents of the three diterpenoids in processed E. lathyris seeds were 3.435, 1.367 and 0.286 mg / g, respectively, which decreased more sharply than those in unprocessed E. lathyris seeds which were 4.915, 1.944 and 0.425 mg / g, respectively. The method is simple, accurate, reliable and reproducible, and it can be applied to control the quality of unprocessed and processed E. lathyris seeds.