Abstract: Monocrotaline in Crotalaria pallida seed was extracted and determined by ultrasonic extraction and gas chromatography -mass spectrometry (GC -MS), respectively. Optimal extraction conditions were obtained as follows: extraction temperature 75, extraction time 60 min, extraction power 255 W, solid-liquid ratio 251(VM). For the GC-MS determination, separation was performed on DB-5MS column(30 m0.25 mm0.25m), the injector temperature was 250益. The mass spectrometer was operated in electron ionization mode with impact ionization voltage 70 eV, a transfer line temperature of 280, ion source 220, and selected ion monitoring (SIM) mode. The characteristics ions m/z120 and 236 were used as qualitative ions, and m/z 120 was used as quantitative ion. The proposed method showed excellent linear correlation coefficient (R20.9994) between the concentration (0.10~2.00g/mL) and peak area, and the LOD was 0.05g/ mL. Precision and recovery tests were made with three samples and the results showed that recoveries was 96%-112%, RSD was 3.7%-6.7%.